Vista.
Where can i start.
Security isn't as tight, but count the number of viruses out there for windows, count the number out there for mac and you will see a huge difference. Chuck yourself a decent firewall and/or anti virus up there and be careful where you surf and this isn;t a factor.
Vista is so damn easy to use compared to OSX imo, the file system is simple, the way to access them is easy, the file types are pretty much universal across the whole system...the whole thing is brilliant.
For gaming, it is ace. Unlike XP 64 it has great driver support on vista 64 and yes, vista does use a fair chunk of memory, but if you have a gaming rig, 0.75-1GB isn't going to eat all that much up.
Vista looks brilliant, again, in my opinion. Visually it is simple to look through it and yet at the same time smooth and beautiful.
I could carry on but those are what i would say the main points which is why i prefer vista more. It got given a bad rep when it was first released because of 2 things:
1. it was fairly buggy.
2. People were used to XP, went on vista and rather than giving it a chance went OH MY GOD!!! DIFFERENCE freaked after an hour of trying to use it and not noticing its simplicity and from then til now have bad-mouthed it. Ask anyone using it now and they will say, hands down it is better than XP and is better than all other operating systems microsoft has ever produced, with the possible exception of windows 98