I'm currently expanding my 2x4 cage to a 2x9 (in an L-configuration) for my girls and I've decided to go with the long and narrow design for several reasons. The first is for space conservation. If you have a large square cage that is lifted off the ground, it needs to be away from the wall so you can access all four sides for cleaning and picking up your pigs. If you had a large square cage against the wall, you couldn't reach all areas of the cage. My L-shaped cage will fit perfectly in the corner of the room and I can clean it without any problems.
My cage is elevated but if I had the cage on the floor and was able to crawl in and sit with the pigs, then I'd prefer a square. But for me this is too much trouble and I don't have the room, so I prefer the narrow cages.