It depends actually.

In a game like Skyrim or a lot of linear RPG's that change appearance based on outfit, I'll typically grab the best due to just how big a difference there is.

In a game like White Knight Chronicles where the differences between the three top tear armors was pretty small, I'd go for looks.

We need more games to go that route where you have multiple sets of armor at similar or Identical stats with different looks, or the ability to craft your armor to look different yet retain the same effects to add variety.