I think you will be happier with the xbox version if you are going to be using a 260m. I'm sure that card will meet the min specs, but you most definitely will not be able to crank up the graphics all the way...in fact I doubt you will be able to get close. As for whether the experience with the 260m will be better or worse than the xbox (my
GUESS is worse)...no one can say for sure atm.
My opinion is: If you don't have a higher-mid to high end gaming machine (or just outright prefer pc gaming) go for the xbox version...you will (at least I would) be happier not knowing what you are missing (I hate looking at the video settings and seeing everything set to medium...it feels like a little piece of me dies...
)
EDIT: I agree with the modding comment...I have modded ALL of the elder scrolls games I have played, and I prefer mods for most of my games...so if you prefer mods then the PC version is, hands down, the way to go!