All I can say is it's a different feel, especially the feel of your seat on the saddle, your legs on the horse (a LOT more leather in western, less in English) and a thinner pad too. And it's different to your horse especially if they have neck reined and are now direct reining.
But as someone showed us in the video (will find it now..) they're the same level of difficulty, but in different ways. A good rider can do both without too much of a struggle.
Have fun though, I enjoy both :) Hoping to get my horse into both too as soon as we get all the basics of dressage down. And by basics, I mean everything from halting to flying lead changes.
EDIT: Here's the video