First person shooters are always best experienced with a keyboard and mouse in my opinion. There is just no comparison. K/M allows for quicker and more precise aiming. They also more realistically replicate real life movement. Joystick controls like on a console for FPSs just don't mix well, I don't care how well one plays on them. From what I hear, many FPSs on consoles actually have an "aim assist" feature which helps players zoom in on targets. What a joke. You don't get that on PC.
That being said, I don't think the platform matters so much. It's the controls that matter. Right now, I prefer gaming on PC. However, should a really good K/M come out for the Xbox 360, I'd be on that really fast. I might not chuck gaming on PC, but I'd certainly spend a lot more time on Xbox.
While I'm on the subject, this might be of interest to many of us here.
http://www.xim360.com
This is a really good K/M adapter for Xbox 360. This is
not the much hated XFPS, but something totally different. The main product they have out now is called the XIM 2. I own one, and it works pretty well. The downside is that it has to work through your computer, which might lead to compatibility problems. I've had several, and it's a pain to work through. Nevertheless, if you get the settings just right you can feel right at home like it's on PC.
They are coming out with a better version soon though, called the
XIM 3. This looks like a much better improvement, particularly because it doesn't need your computer to work through. It works by itself. Looks like they're gonna be coming out with it real soon too.