My preference is for a 2 grid wide cage because you can't reach across a 3 grid wide cage unless it's accessible from all sides. It's a lot harder to clean and change the bedding.
Hmm. Well, I could keep it pushed out from the wall enough for me to fit between the cage and the wall so I could reach. I just want my piggies to have the biggest cage that I can make for them. I might also add either a second floor or a stand so it doesn't have to be on the floor.
I've had a 3x5 before (probably going to build another one shortly) and yeah, it's wide but not horribly so. A lot depends on the type of bedding you use; fabric is easier to clean up than shavings or Carefresh or other types of loose bedding.
My 3x5 is against 2 walls and I can reach all corners. I have to set my hand in the cage, but I do that even with my 2x3 cage. Having a wider cage seemed to make my 4 pigs get along better than a longer, 2 wide cage.
I just put together a 3x4 (and part is a 4x4) and I actually just get in there with them. I love it, I'm not sure they like it at first but it only takes a minute before they are curling around my feet and running under my legs like I'm a living tunnel. I don't have coroplast though, mine is directly on the floor with a shower curtain, towels and fleece.