Really depends upon the discipline, rider, horse, experience, and many other things. If you were comparing for example English Pleasure, with Reining, I'd have to go with the Reining being more of a challenge.
Now if you would compare trail riding- with trail riding in either discipline, personally easier/ more relaxing for me would be western. For one the horses in my barn trained in the english discipline are trained to TROT, the western ridden horses JOG, therefore I post to the trot, whereas I can relax comfortably to a jog. Another reason would be that I can relax fairly well in a good western saddle, and it has more support hold-you-in parts. When riding western I also have a free hand.
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Oh, but cowboys do post and their good at it. Really the only Western discipline I can think of that doesn't post is WP.
I do see western riders post and in a variety of events, but VERY rarely. Generally when warming up. 99% of the riders I ride with, or public trail rides, no one western generally ever posts.