If bethesda's talks with microsoft involving mods are a success, I'd say to stick with the 360. If not, get it for PC (or PS3 if sony agrees instead).
Even if Bethesda is successful in convincing Microsoft/Sony to allow user mods, you still have the hardware to contend with. Skyrim is going to be pushing the console hardware very hard, and it's not likely to have much wiggle room before hitting some hardware limits. You won't see anything like Oblivion's QTP3, Unique Landscape, or RAEVWD mods getting to the consoles for Skyrim... they simply require too many resources that the consoles just don't have. The PC would still be your best option for mods, as you can always upgrade (now or in the future) to meet the mods' demands. Additionally, some special types of mods, like OBSE, OBGE, and MCP, will require a level of access that you will not ever get with consoles.
To put it more succinctly, imagine if Microsoft allowed mods for Morrowind on the Xbox. There'd be no way in [the Deadlands] you'd get http://www.youtube.com/watch?v=2r6hilB6Dlc running on it... while any decent PC in the last 5 years will be able to benefit from many parts of it.