I've DM'd 3.5 but not 4th (but played in it).... but it seems to me like DMing 4th, especially when you haven't played yet would be harder than DMing 3.5.

But maybe that's just me. 4th ed has a lot more of a fine edge of balance. Whereas 3.5 is all over the place, which to me is a good thing. If someones lagging behind its easy to boost them up or change things up. If someone sucks at playing 4th (and yes, I've seen several people suck at it, despite its claims of easiness) then its almost impossible to help them out except blatantly changing their character or giving them extra levels or something which isn't good for group cohesion.

4th ed. is also generally harder to roleplay for most people. Yet another challenge you'll need to overcome.