If you've got a good, natural seat it shouldn't matter what saddle you ride in.
Having ridden both quite frequently, there is a difference in the way both saddles position you on the horse's back.
An english sets you higher on the horse's shoulders and also pushes your hips forward a bit more, whereas a western typically sits you nearer the center of the horse's back and allows you to sit deep a lot easier.
When it comes to cantering in a western, I find it easiest just to relax and roll your weight back onto your tailbone and let your hips absorb the motion. In an english I like to rise a little forward and grip more with my legs (which isn't as simple in a western because there is alot more, stiffer leather between you and the horse).