If you can afford it definitely go for the stronger snake/vermin proof wire mesh. It's worth the extra cost for the initial outlay, don't want to be feeding rodents your chook feed or replacing chooks that Mr. Fox has run off with or losing eggs to snakes.
Mesh on the flooring so that nothing digs under and gets in, topped with a deep litter, so the chooks aren't walking on bare mesh and damaging their feet can result on sores and calluses on their feet if there is not much access to free ranging.
