Basically I trying to determine which of these OS I should choose from. I found a lot of 32-bit vs 64-bit threads but not 64 vs 64 threads.

I have an AMD Athlon 64 bit processor and I am planning on upgrading my memory between 5-8 gigs. I am not going to be hooking up any peripherals like webcams, printers, etc to the system so I don't need to worry about many drivers.

The main purpose of this system is for content creation, programs like Maya, XSI, RealFlow and others which are processor and memory intensive and also are available in 64-bit versions. I might use it for games (Half Life 2 or other Steam games) but thats not a big concern. So I've done research on 32-bit vs 64 and I know what runs and doesn't run on 64-bit system and don't need advise on that topic.

So between the two 64-bit OS which do you feel is faster, more stable, has better support, or some other factor I haven't mentioned based on what I've written above.

I would stick with xp; personnally i think vista should of been pure 64 ; but its not; so xp is best choice. The upgrade of 5 to 8 gigs dictates that you must move to a 64 bit system; however, if you are on an existing 64 bit system using xp, then i would still go with xp as your software may not work (and commonly does not) with vista. Also., xp adds perfromance enhancement at lessor cost vs vista ( in other words even with increased memory; you would get same performance with xp computer at half the memory)

You will find far more support and software/hardware for Vista 64 than XP 64 - just go out and look for yourself to confirm this.