I think it all depends on the person. I've seen some people train legs in my gym with such intensity that I doubt there are many people that would be able to keep up with them, and on the flip side, I've seen powerlifting routines on youtube that make me feel sick just watching
What you need to remember is for a decent bodybuilder who trains hard and properly, they could cope with a powerlifting program no problem in terms of the sets and the rest periods etc, as their endurance will more likely be higher than the powerlifter. Perfect example was when I trained with Scott and Dom, when they did my routine, the weights went out the window and it was purely just about trying to get through the sets, because they didn't have the endurance, they still did brilliantly and did it without having to give up, but they'd be the first to admit it was a big struggle. Yet if I had their workout, I'd be fine with the rest and the structure of the sets, but the sheer weight I'd be lifting would take it out of me beyond belief and I'd find it ridiculously hard because my working sets would be so much higher than I'm normally comfortable with, so that's where I'd be struggling.
It's all relative. I think to say one is more difficult than the other isn't really possible as it all depends on the person training. I think you could argue for both all day and still not have a result