Since the exact specs that the game is going to require for the PC version have not been released it would be impossible to say which would perform better at this point but since the game will allow modding if the 260m does fall within the recommended specs once they are made official then the PC version is the way to go !!
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!
Dude, the 260m is just a downclocked 8800gt, no way will it be worse than a 360.
Yeah maybe, that was just my guess. With the xbox it is not just about power though...things have really been optimized for it. If you were to take a computer with the exact same power as an xbox you would be hard pressed to find a game that ran as well on it (the computer) as it would on the xbox. Anyway...I don't really know, it was just a guess. You are probably right. *couple minutes later* Hmmmm, does look like the 260m has some pretty good benchies...yeah, it will probably be great.