There is actually a lot of written material on the topic of the European medieval diet. You need to bear in mind that the Medieval period lasted for up to a thousand years (c. 500 - c. 1500 CE) by some people's reckoning so this quickly written answer will only be an approximation of the difference between noble and peasant diets in the period!

The European medieval diet was largely determined by social class. For the majority of the of the people, peasants, a large portion of their daily diet was made up of grains such as wheat, rye, oats or barley(carbohydrates). The grains were boiled whole in a soup or stew, ground into flour and made into bread, or malted and brewed into ale. Estimates from the late Middle Ages indicated that a gallon of ale a day was not unusual, but the actual alcohol in the drink was low. Protein was usually provided legumes such as beans, peas or lentils, fish where available, or on very rare occasions, meat such as poultry, pork, or beef. Additional nutrients were provided by seasonal vegetables and fruits. The peasant's diet rates high on modern nutrition standards. But seasonal fluctuations in food availability and poor harvests often caused long periods of very poor nutrition.

This quote from Singman is found on this webpage that has tables exploring the nutritional value of peasant diets during the Middle Ages. TL;DR for the tables: if famine didn't happen to you, you would probably have a relatively bland diet with a nutritional value that would make a modern dietician spaff their pants.

That isn't to say that anyone in Medieval Europe thought that greens were good for you. In the Boke of Kervynge from 1508, the prospective butler is told:

beware of grene sallettes & rawe fruytes for they wyll make our soverayne seke / therfore set not moche by suche metes as wyll set your tethe on edge therfore ete an almonde & harde chese / but ete not moche chese without romney modon.

This extract is basically telling the servant not to serve their noble salad or fruit (as they will make him sick) but rather to serve him almonds and hard cheese. It is also prescribing that the noble not eat too much cheese without some sweet wine to wash it down with. Whilst this dietary advice looks hilariously bad to a modern reader, it is important to bear in mind that it was not given as part of a nefarious plot on the part of the peasants to strike down the rich with diabetes or CHD. Rather it was due to top-down constraints western European cultures put on diets that were to be enjoyed by people of specific social stations.

What you can essentially guarantee is that on the whole during the Middle Ages that you would have a lot of quite fat older nobles and muscly younger nobles (due to the opportunity to play sports, density of sugar, protein and fat in the diets they enjoyed and the crash in metabolism after a certain age) and a lot of wiry or malnourished peasants (dependent on how well farming went that year).

If you want more detailed information on this topic, I suggest you read:

*M. Adamson (ed.), Food in the Middle Ages;

*B. Henisch, Fast and Feast;

*T. Scully, The Art of Cookery in the Middle Ages.

EDIT: removed something

A peasant's daily fare consisted primarily of grains, porridge, and rice. There were some legumes in there as well as some animal products, but animal products - especially meat - were quite expensive even around the end of the Medieval era. If a peasant did have meat it would be in the form of sausages or leftover parts. They boiled all of their food as to not waste any calories, but they were left vulnerable to numerous vitamin deficiencies as one would expect from a diet so high in carbohydrates and not much else.

Nobles would have heavily spiced and decorated meals; they could afford fresh vegetables and meat. In contrast to peasants, they would fry or grill their food because they could afford to waste calories in a way that peasants could not. Of course we now have this notion that frying is unhealthy in comparison to boiling, but on the other hand wealthier folk did not commonly suffer from vitamin deficiencies, owed to their relatively balanced diet.

Sugar did become a large issue, but not until the 19th century or so. In the era we're discussing - the Medieval era - sugar did not see any use as a sweetener. Sugar started out as medicine around the 13th century; it was widely believed that sugar would sooth coughs and contribute to the health of one's liver and lungs, among other things. This medical use was part of the reason for its high demand and equally high price. Around the 15th there is evidence of sugar being used in cooking, however not as a sweetener but as a spice. It would be used in extremely sparing amounts to add to the flavor profile of a dish as opposed to making the dish noticeably sweet. Even then, it was an ingredient reserved only for the rich; in the 15th century, only nobles could afford this exotic ingredient that had to be imported from afar, and it was not used in amounts that would affect one's diet or health one way or the other until the late 1700s or early 1800s.

If you asked people in the 15th century, nobles had a healthier diet. As I mentioned, they spiced their foods extensively. Spices were seen as medicine just like sugar. See another post where I discuss the theory of humors and how spices were believed to cure certain illnesses. Just like sugar, the medical uses that spices had meant that they were in high demand, and out of a peasant's reach. If we're contextualizing this historically, the vast majority of people in the 15th century would agree that nobles had a healthier diet because they had access to these spices they could use to treat and prevent illnesses. I would argue the same today because nobles weren't exposed to as many vitamin deficiencies and nutrition-related illnesses as peasants were.

Sources:

Jeffrey Pilcher, Food In World History

Paul Freedman, “The Medieval Spice Trade,” in Jeffrey Pilcher, ed., Oxford Handbook of Food History

Sidney Mintz, Sweetness and Power