amund: One thing to consider is how good the support in the games you want to play for controllers, Xbox controllers are pretty much standard
and especially for older games. If you don't want to mess around with additional software I would go for Xbox. Most games work just fine with mouse and keyboard so you can try that first.
Otherwise I might agree, but that bolded part isn't really true. XBox controllers (and other XInput controllers, like the Logitech F310 and such) are good for NEWER games, while older games rely on the older DirectInput standard.
While it is usually possible to customize the Xbox controller to work passably also with such older DirectInput games (sometimes with extra software like XPadder or whatever), it doesn't fully work with them out of the box. Logitech F310, however, is better in that regard as it has an optional "legacy" DirectInput mode as well, for those older games.
Anyway, in the big picture I'd say that doesn't matter that much because usually the older DirectInput-era PC games were really optimized for mouse+keyboard, not really gamepads. Sometimes they are optional, e.g. Prince of Persia Sands of Time works quite well both with mouse+keyboard or an older DirectInput gamepad, it is mostly a matter of taste which one you prefer with that particular game.
So in that sense buying a XInput-only gamepad like the Xbox360 controller is quite fine, I guess. It will be used mainly with newer (like 2008 and newer) PC games which were specifically optimized for it. Older games would then be played mostly with mouse + keyboard, except for maybe space/flight combat sims and racing games of course.