strategy: which is better

Usually it's better to capture towards the center of the board. However, there are some exceptions, such as in the Ruy Lopez exchange variation. (In this case, it's for tactical reasons.)

I'm not exactly sure of the reason for generally capturing towards the center - Perhaps it's because there is usually more mobility for your pieces near the center of the board, and having more pawns near the center can better support your centralized pieces.

Usually it is better to capture toward the center because that helps to control the center, plus rook pawns often can only draw in an endgame. However, there are two exceptions that are common and important: (1) In double e-pawn openings (especially the Exchange Ruy Lopez and Petrov's Defense) it is better to capture QPxBP instead of NPxBP because the queen can benefit from the ability to (or needs to be able to) jump forward on the half-open d-file. (2) If you would suffer a "queen slap" (QxQ+ ...KxQ) after playing QPxBP, then NPxBP is better, because a "queen slap" would prevent you from being able to castle, which is a slight (sometimes important) drawback. I can post examples if this description isn't sufficient.

Forget about following any positional theory for now. Making moves without a clue as to why your making them is just silly. Once I decided that I would start playing by instinct and tried as hard as I could to keep everyone of my pieces safe from getting captured I quickly became a 1600.