There's a lot of ground to cover here - I hope this answer will make some kind of sense by the time we get to the end.

3 Problems with your Question

First, you're asking about the 6th century BC, but the Persian Empire doesn't actually exist for the first half of that century. Around 550 BC, Cyrus II conquers the heartland of the Medes, which can be seen as the rise Achaemenid Persia (though this is a slight simplification as Cyrus was a Teispid). Throughout the rest of the century, the new power of Persia keeps expanding, absorbing first Lydia and Babylon, and then most of eastern Iran, the Levantine coast, Egypt, parts of the Indus valley, and Thrace. Persia is not a constant presence, but a growing world empire, and the reach and nature of its dominion are only just taking shape. For the sake of this answer, I will assume you were born at some point in the reign of Darius I (c. 522-486 BC), which is generally considered the period when Achaemenid power solidified.

Second (and you probably know this already), you could be living in a Greek city state and in the Persian Empire. The Greek cities of Western Asia Minor, those on the south coast of the Black Sea, and the settlement at Naukratis in the Nile Delta are all part of the Persian Empire by the end of the 6th century BC. This complicates your question for a number of reasons, but it also helps. We can make a better comparison between places to live if they both belong to the same cultural zone but exist under different political circumstances.

Third, the "middle class" is a modern concept. The Ancient Greeks did not know it; while I don't think we have enough information about the socio-economic or political thought of the Persians, I doubt they knew it either. It is true that the Greeks idealised the philosophical concept of the middle as a superior "third option" that avoided unhealthy extremes, but this concept did not map onto any particular social, economic or political group in society. Indeed, the ideal of the middle was malleable to the point of meaninglessness. Aristotle once refers to a regent of Sparta as a member of the "middle" because he wasn't a king. In practice, the Greeks saw society as divided into two groups: the rich and the poor. The difference between the two was that the former owned enough to live a life of leisure, while the latter had to work to get by. In this worldview, every single person we would consider "middle class" was considered one of the poor. This category may have been very broad, ranging from penniless day labourers to well-to-do independent farmers, but it wasn't subdivided in any meaningful way (except, of course, for the strict distinction between free citizens and slaves).

So, when we're talking about a "middle-class man" in the 6th century BC, we have to bear in mind that we're applying an arbitrary modern distinction onto a past that didn't recognise it. Generally, the existence of slavery and the peculiarities of a subsistence economy led to a system of labour relations that doesn't quite overlap with what we know today. For the sake of this answer, I'll assume you are either an independent small farmer or a specialist craftsman - categories that would put you above abject poverty, but would still require you to use your own labour (along with that of your family and slaves) to survive.

The Nature of Persian Rule

This is where we get to what you're really asking: are the conditions of life in the Greek city-states c.520 BC fundamentally different from those in the Persian Empire? Would the average man of modest means notice the effect of his political situation?

We should not be distracted here by Greek rhetoric about how all the subjects of the Great King were his slaves, and how the barbarian ruled his realm with an iron fist and with the arbitrary cruelty that was typical of an oppressive despot. This is mostly a matter of propaganda. In fact, we have the counter-propaganda too: Persian reliefs from Persepolis show that the Persians themselves liked to see their empire as a collection of peoples cheerfully joined in the worthy task of supporting the righteous rule of Ahura Mazda's representative on Earth. They paid tribute, and in return they received peace and justice from the king.

In practice, all the evidence we have suggests that the Persians liked to rule, as most empires do, by affirming and preserving existing administrative structures and political systems in return for tribute and loyalty. In other words, they were happy to let their subjects do their own thing as long as they paid tribute every year and sent as many troops as the king needed. The famous Cyrus Cylinder confirms the privileges of the old Baylonian priesthood; Darius' self-representation in Egypt casts him as a traditional pharaoh, honouring and paying for the upkeep of the old cults and temples. In Asia Minor, the Greeks were initially allowed to keep the tyrants that ruled their cities. When the Ionians rose in revolt in 499 BC and deposed their tyrants, the Persians, upon crushing the revolts, decided to let them keep their new democracies, too (Herodotos 6.43.3). As long as the tribute kept coming in, the Persians were cool with whatever.

It follows that we shouldn't think of the Persian Empire as a despotic, barbaric realm in which all freedom of thought and enterprise was stifled. Rather, it was an agglomeration of semi-autonomous communities that were free to do more or less what they wanted as long as they paid the tribute, served the Persian war machine, and recognised the complete subordination of their foreign policy to the interests of Persia. Individuals within the empire may not have noticed its presence very much, unless the tribute came partly out of their pocket, or if their area had received a Persian garrison.

This form of rule, coupled with the Pax Persica, meant that the Greek cities of Asia Minor actually flourished once they had fallen under the Persian sway. Possibly their integration into the imperial system opened up new markets for their traders; certainly their connection to the old civilisations of the East generated a boost in intellectual development, with the first medical and ethnographic treatises, the first world maps, and the first histories ever written all originating in late 6th century BC Ionia. Herodotos (5.28) notes that Miletos, which had suffered endless internal strife before, became "the jewel of Ionia" once the Persians guaranteed the position of its tyrant Histiaios. A similar development may well have taken place elsewhere in regions that were previously unstable. In regions that had previously enjoyed unity and stable rule (like Egypt under the Saite dynasty), life would have gone on more or less as before.

So Where Would You Rather Live?

So let's get down to brass tacks. As a man living in modest comfort, where would you be better off? As should be clear from the above, it really depends on where you are and what you do for a living.

As an independent farmer, your life is likely to be the same all over. Both the Greek world and the Persian Empire knew a small leisure class of large landowners, but the small farmer was known to the Greeks by the late 6th century BC, and no doubt he would have been seen elsewhere too. And the life of a small farmer was hard work wherever you happened to live. The only difference between some (but by no means all) Greek city-states and most of the Persian Empire is political representation. In states like Athens, you'd have a vote in the Assembly, and a share in the political processes of your community. In most Greek cities you'd enjoy certain rights as a freeborn citizen. These things were still in development, and even many Greek states were run by tyrants, or by oligarchs, like the cities of Phoenicia, but still, if politics is your thing, you might be happier in Athens or Cyrene than somewhere in Mesopotamia.

If you're a specialist craftsman, you really want to be in the trade network, regardless of where you might be. From the Phoenician cities on the Levantine coast, a centuries-old trade web spread all over the Mediterranean, and you could profit from this web whether you were on Samos or Cyprus or Sardinia. The example of Ionia above shows that being part of the Persian Empire doesn't seem to have hampered trade - indeed, it may have boosted merchant activity as more regions became pacified. By the late 6th century BC, you could also benefit from the new network spreading east over land - the Royal Roads that connected the western administrative hubs of the Empire to its core in Persia. Where you obviously don't want to be is "off the grid", in remote semi-autonomous parts of the Persian Empire that saw limited economic activity. If you were in the old Assyrian heartland, for instance, or in some distant area of the Iranian plateau, there would be little else for you to do except sell to your local community. Far better to be in one of the wealthier, better connected parts of the Empire, like Ionia or Lower Egypt.