I agree with Burke. I have had about everything and the z400 by far took the most abuse. I had it piped,jetted and filtered so it wasnt crazy but it was a good machine. But then again I've heard a 400ex can take some abuse too.
Like some has previously said it doesn't matter what you get because you will have to work on it at some point. The good thing is there are sites for just about every bike to walk you through every problem you could have.
Personally I like 4 strokes for trails/dunes and just riding around and having fun. When it comes to drag racing though I'll take a banshee any day.