Lots of different designs when it comes to seats. Naturally a low profile (less padding) gets you closer to the ground. When it comes to gaining solid footing, I think the dimension at the nose of the seat is far more important than the overall width.
It's all going to depend on where that measurement is taken. I'm assuming the 15.5" is at it's widest point. The wider the seat generally means your legs will be spread out a little more and it will decrease their bend at the knees. As hoghawler pointed out, if it's narrower at the nose, you'll more likely to flat foot it at a stop.