Vista for me.
I have used Windows since the 3.1 days and I have been through all the iterations from then to Vista. Now, XP is a fine OS, no problems, so on and so forth. And so is Vista.
I believe them to be very comparable. In terms of compatibility XP has just as many old programs that do not work as in Vista. Now, personally I have only had one issue in regards to compatibility and that is that one game I bought(Broken Sword) did not work. Everything else has been flawless.
There is the problem of better driver support on XP over Vista. It is true BUT that is not an OS issue, it is not Vista's fault. The manufacturers and developers have had ample time to code proper drivers, it is not as though Vista is stopping them at every term, Vista has NOTHING to do with it.
In terms of asthetics, Vista has XP beat hands down, the interface is cleaned up and uncomplicated, although I do not like that some advanced things are hidden better, I do think it simplifies it for the average user. And if you do not like how Vista looks you can just as easily convert to a XP type layout. Aero looks nice, plain and simple. Makes working a bit less dreary.
Vista also has much better support from MS than XP did back when it was out. The automatic updates, although I do not have set to auto, make it really simple to down load the latest updates. Which come pretty often, more so(or at least it seems) than XP, but for sure with more ease. And when you install updates you have to reboot much, much less. Unlike in XP where with every patch or so you have to reboot.
In terms of speed, the depreciation in Vista is negligible. Many people argue that having 2 GB of RAM for Vista is a good amount, and I agree. And some may argue that XP needs less and that is also true. But if you play games you know that games become more and more power hungry so you chances are you will have 2 GB or more in any situation, and there is no added cost to Vista ownership. I have not had any issues in terms of performance when I switched to Vista from XP.
Vista does have some issue, namely the file transfer speeds. They were mostly fixed in SP1 but they are still a bit behind XP, but that is a small problem I can live with. Also, Vista has crashed much less on me than XP ever did, it is stable as hell.
DX10? Yeah, it is nice but that is all.
Many of you keep talking about upgrading to Vista, and I agree if you are paying for it then YES, it is not worth it. Was XP worth the several hundered dollar upgrade? NO, nothing really is. XP looked better after a couple years, no? Of course a new OS is not going to be worth what a company charges, it rarely does. I mean really, $100 for some new feature that adds little to the overall experience, no one cares. But, the arguement here was mainly on an OS to OS comparison in terms of features, and in that case I think Vista wins hands down.