first xbox, why? cuz it's how i did it for oblivion and morrowind, and something about it is so much more relaxing. then when i get bored of the vanilla version, play it on PC, as i know my 6970 can handle it.
360 & then PC. I much prefer playing on console but I know that (hopefully) after my first play-through, I'll want to mod some things. That and my PC is a beast, so I'll enjoy seeing the top-spec graphics, and graphics mods.
I had to play Morrowind on the xbox and Oblivion on the 360 because I never had a decent computer. I always had to watch my friends play awesome mods on the PC though.
I finally have a great gaming rig so definitely the PC.