I've ridden just about everything under the sun. On the western side of things, I've ridden reiners, Western Pleasure horses, Horsemanship horses, barrel horses, working cowhorses, gaited horses, and trail horses. On the English side, I've done dressage, hunters (on the flat and O/F), have ridden a country english pleasure horse, done a bit of endurance, and am conditioning my TB for eventing. The one thing I have not tried is Saddleseat.
As for differences, every discipline in itself is so different from the next, yet they all are based off of the same principles. Posting is, of course, incorporated into many English disciplines, whereas Western riders in show do not post. Snaffle to curb bit is a change, though not so much if you're coming from the upper-level dressage world. The leg and seat aids remain the main source of aids, and in both English and Western, the best horses don't rely on rein aids.