First, Slackware is NOT for "experienced" users only. Slackware is pretty much like Ubuntu now. Second, XP is better than Vista. Who said Vista has better security than XP? Google for Vista exploits, I haven't googled but I'll bet you more exploits were found in a short range of days for Vista than XP. But anyway, I'd pick Ubuntu over any OS. Ubuntu is awesome, as easy as it seems, it's also for "experienced" users too, which is why I love it. It has some challenges for me since I'm on Dell Inspiton 1501, it's a 64-bit laptop and Ubuntu has always been a challenge for me on 64-bit PCs/Laptops. Flash doesn't work the best on 64-bit and I'm disappointed about that. It's really Adobe's fault anyway, I think they're trying to come up with a solution for that. Whoever said applications are 'limited' on Ubuntu. Wrong. There are so many applications that are pretty much like Windows' applications and don't forget you have WINE, you'll be able to run *MOST* of Windows' applications on Ubuntu.
Overall, I'd pick Ubuntu over anything.