Well, I know some western ridrs are pretty hard on their horses, but I also found that the more I watched the more I realized they weren't being hard, it was all a sham. Like the reins they used made a cracking sound. I tohught they were hitting their horse really hard, but it turns out the reins were made so that even with the slightest tap it cracked. More of a motivation. I also know that a lot of people misuse standard western equipment, such as the spurs. Most western riders use spurs to jab into the hroses sides, when in reality you are meant to roll your heel down the belly. Just slightly sharper.
English is also very demanding for the horse. Horses do not usually jump unless they have no other choice and like some others said the agility and speed behind it is very demanding.
I don't think either is harder, I think western just seems that way because it's more of the "rough and tough" style.
Also, I would like to add I am mostly a western rider btu I sometimes do Hunter jumper on my mare, Annie. On the other hand, Jester is incredibly numb to your commands. I find myself yelling in the ring for him to pick up the lope. Eventually he will, but he needs the vocal and physical commands. I found that some other horses are like that as well, where others do not. Jester is just lazy.
I believe it depends on the trainer and the horse. I am more firm with my horses, but I love them to death. Sometimes you have to remember there realy is nothing you can do within your own physical range to really hurt them badly. I mean, if you're abusive then obviously it's a problem, but sometimes a horse needs a good motivation. But you have to know how far is too far before you do it.