A two piece is easier to install, by far. If you line it up just right there should be little to no difference between a 1 piece and a two. Think of it like a bikini, would you rather see Jessica Alba in a 1 piece or a 2 piece?!!
But seriously, a 1 piece will have better flow.
I went with a 1 piece b/c I didn't want to deal with trying to get a 1 piece in.