Hiya! I'm not entirely sure that this was the kind of contrast you are making it out to be. More often than not, an individual ruler did not really have a choice to pick "personal unions over consolidation of power," because it is really hard to define what consolidation would imply, since the concept did not really exist. Most of the time, personal unions formed as the result of inheritance or some kind of succession conflict (France and Navarra, England and Normandy, France and Burgundy, Aragon and Castille). But this isn't quite like inheriting your dad's old car. You are in short, inheriting a political entity, with its own laws, obligations, privileges, bureaucracy, taxation system, military system, aristocracy, conflicts, and factions. Sure, they aren't nation-states in a modern sense, but you can't completely eliminate or change the existing institutions without often violent consequences. The king of a really big country might be able to modify the administration of a city or small province, but most monarchs during the medieval and early modern period were no where near powerful enough to enforce widespread changes in another kingdom. I wouldn't characterize this as some kind of optimization choice between personal unions and "consolidation." A monarch would likely want a system that puts all the economic, political, social, and military power in his hands, but at the end of the day, they are going to need some kinds of institutions to exercise this power and there are other political actors present. Just because they are king doesn't mean things magically change to work for them.

I hope this is making sense, these processes often take a long time, and there is a huge amount of path dependency here, the weight of these institutions. "Consolidation" takes time :) and more often then not, it still results in conflict. To give some examples, the Kalmar Union (Sweden, Denmark, and Norway) was not always in effect and never unified at all. The Plantagenet kings of England spent hundreds of years trying to keep their continental possession independent of the Capetian French Kings.The United Kingdom took over a hundred years after the personal union of Scotland and England under King James to start developing uniform institutions, and there is still pushback on this front. The Kings of France considered Burgundy a vassal fief of their kingdom, but when they inherited during the Hundred Years' War, they were entirely unable to consolidate it into the royal domain, as the Burgundian nobility resisted. It became the domain of a younger son of the King, and was only integrated into France over a hundred years later, when Louis the XI invaded the duchy upon the death of the last Duke.

I'll try to briefly touch on your specific examples, but I can't necessarily go into depth on them like I can on early modern or medieval examples (if you have more questions on this period, please let me know, I am happy to discuss further!). I'm not entirely sure what you mean by unification in Iberia, as the marriage of Ferdinand and Isabella formed a personal union, and not a very stable one at that. The Habsburg monarchs ruled Spain through this union of Castille, Leon, and Aragon, as well as maintaining privileges for several other regions. Castille was technically the richest and most powerful of these kingdoms, but the unification of "Spain" only happened during the War of Spanish Succession (1700s), when the Bourbon claimant, Philip V, abolished many of the institutions and privileges of regions that opposed his succession (worth noting that he did this by force of arms, and with the backing of his grandfather, the King of France). The unification of Germany and Italy both fall after the Napoleonic Wars, and I think were in many ways driven by them, but you will need someone more qualified to write on this.

I know this may not have been the answer you were expecting, but let me know if you have any follow up questions or want more information and I will try to help!!