I'm a PC guy, myself, but I think the major issue comes back to what's already been said: what are you comfortable with? It sounds like you've been using a Mac for a while now, and are probably pretty used to it. So, there are probably some things you need to ask yourself about it, maybe like: can I find things easily and quickly with the current set up? Do I find it satisfactory (does it do everything I want it to, or are there features that I would like that are lacking)? Does it run the programs/games I want it to? Have I had any major problems with it? Would I mind trying something new or different? How much would it bother me having to spend a little bit of time learning the differences?
Anyway, those are just suggestions. You might change some of the questions, get rid of some, or add others. I think after asking some of them, you might have a better idea of what it is you want. As has already been said: they both have advantages and disadvantages-- Macs seem to lean more toward being good at art and graphics type things, and PCs more toward games and other widespread programs. (I would say something more specific, but I can't think of any at the moment...)
I've been using Vista for about a month now and not had any stability problems with it or anything, but I don't want to resurrect that issue-- it seems to have been fairly well settled.
I don't like the one mouse button on the Mac. That's my biggest issue with it. Other than that, I think I would have some difficulty finding things for a little while, but you get over that fairly quickly if you use it almost every day.
Anyway, remember that the biggest questions are what you're going to use it for and what you want/feel comfortable with.