Round are better for just installing them and never thinking about them again, but if you are meticulous about your sled the squares are much better (and doubles are better yet) because they support the stud much better. The reason i say you have to be meticulous about your sled to see the benefits is because you need to keep the square backers aligned. If they turned side ways (or diagonal to the direction of rotation of the track) then they dont have much support and the corners of the backer can poke through the track causing a rip-out. If you stay on top of them when they are new and torque them properly right off the bat and retorque after you get a few miles on it they pretty much take a "set" and you just have to check them every so often to make sure they are still aligned. Use double backers and you never have to worry about them turning, and if one stud hits a snag it has twice the support to avoid a pull through. Bent studs are WAY better than pull throughs, so support is everything!