I'd say the pc version:
1) Upgrades, patches, mods
Bethesda have already stated that upgrades etc will be available on Xbox Live! This would also include patches if needed, although it should hopefully be polished enough already on the 360.
2) Depending on your pc it will be better than the 360. Just think how good it will look in a few years time when the 360 starts to age.
"Depending" is a very loose word to use here, and it can be used to validate either platforms point. On the 360 you're guaranteed to have it run in a specific way, as it was intended by the developers. The 360 will have the highest quality right from the beginning. If your PC isn't up to it, you'll have to spend however long it is before upgrading playing the game in a lower res, with lower graphical settings, therefore losing out on a lot of the technology the game has to offer if your PC can't handle the highest settings. The Xbox 360 will be ready to take advantage of the highest and latest technologies right from the start. By the time the 360 ages, the next version will most likely be in development or on it's way out to the shops. Obviously my above statement relies heavily on the fact that you'd need a HDTV in order to get the best graphical quality, but even if you hook it up to your PC monitor with a VGA cable, the quality compared to your average PC will be far better. Like I said, "depending" can run both ways 
3) Cheaper.
Only marginally, but if you look at it in terms of the hardware required, if you end up spending extra cash on more memory, or a new graphics card just to be able to get the best out of the game, you're not saving any money at all. The most you'll end up forking out for your 360 will most likely be £65 for a HDD and £15 for a VGA cable. And then you're guaranteed that the game will look as intended.
4) I'm sure it will be nicer to play with a mouse & keyboard. I hate using pads for first person games, not good.
I agree that keyboard and mouse is great for games, but it's third person and doesn't require as much accuracy as a first person shooter for example. Also, generally in these types of games, your reaction times to the environment around you is significantly less than that in an FPS. So I don't see the fact that it's a pad will make much of a difference at all.
Add to that the fact that you don't need to be sitting at a desk to play and enjoy the game, so you can crack open a beer, grab some snacks, and veg out in front of the TV with your shiny wireless pad and game on. Oh yeah, and playing a game on a console hooked up to your TV is far more sociable if you have other people with you, as they can all veg out too, while they watch you play, rather than having to hunch over your desk / chair 
It's due out 24/3/06.
