score:7
It doesn't matter in the least. You can choose on any basis. You can flip a coin. You can choose according to the wind. It needn't be the same basis each time.
Your rights in other EU countries other than Belgium and Finland are the same regardless of whether you show a Belgian or Finnish passport, and even after entering with one passport you can always show the other in the unlikely event that you should need to prove a specific nationality, for example if you were being expelled.
Upvote:4
Given how little it matters, I suggest whenever possible picking one passport for actual use during a trip, and keeping the other as a backup in a more secure location, such as an inner pocket. If you are, for example, starting and ending a trip in Belgium, use that passport for the whole trip.