@gates
Gates are just different. They may be better or worse depending on person and characters you play, but you should understand the differences.
Square gate:
Octo gate:
First thing is first, each zone of the images is a different approximate trigger range for the stick to be indicating directional motions to the console. You can see that the octo gate has significantly less corner area, which means it is harder to go from neutral->diagonal. For precise movements, this can sometimes suck. The neutral area is also larger on an octo gate, which is actually nice in some games (KOF XIII) which encourage you to return to neutral for different movements.
I started with a square gate, didn't like not being able to rest my stick in the up/left/down/right positions, tried an octo gate, and have been using one ever since. However, my recent frustrations with not being able to hit diagonals perfectly have made me want to go back to a square gate.
I recommend trying both. Gates are cheap and easy to install. Give yourself at least a month using each gate.
edit: Oh yeah, another thing I was gonna iterate on:
Because of the curvature of the octo gate, it makes circular motions (quarter, half, full) much easier because you can swing along the edge more easily than on a square gate. And for the square gate, because you don't slide as much, it makes DP motions easier. Since I play Juri in SSF4 who has -no- DP motions, the octo gate is a great choice. I just have problems when I venture outside her