Well, I'm working on a light civilization game, and I'm going to do something different: hexagon cards. I like some of the mulitfunctionality of cards in San Juan and RftG, but I want something a little different. With hex cards, I can have the civ actually be
built (space-filling shape) and have cards influence neighboring cards.
From the standpoint of ease, squares are much easier. My game is going to run me through a lot of ink and cardstock, due to shape and size. If you want a modular board, though, either could work. Think of it somewhat like a retheme of
Fjords, at least for the tiles. Yes, you probably want larger, but that's not the point; that's just a matter of scale.
Alternately, you could do two versions: one with square tiles, one with hexes.