I think the original question needs to be clarified a little bit...
If you consider "winning" on WWTBAM to be making it to millionaire, then it's harder...
But, the pace is much easier, less pressure, plus you get phone a friend, etc. So, in regards to the pressure, it's easier.
But, I've had a feeling for quite a while while watching the show that they try to pick questions that fall well outside your area of knowledge, based on what they know of you. Well, after about 32K, that is.
But, regardless of lifeline usage, you ultimately have to answer EVERY question correctly. Whereas on Jeopardy, you can just plain not answer (as well as answer wrong).
Personally though, I think I'd be a much stronger contender on jeopardy than on who wants to be a millionaire. Maybe that's why I think of jeopardy as easier.