I have played both versions. On my rig below and on my 50" plasma. I find the visuals of both to be great. I played more on the xbo360 because it was enjoyable to play from the easy of my lazyboy for long sessions. I really like the 360 wireless controller too. The PC version is probably still cheaper. For me the choice boiled down to mechanics since both had beautiful visuals and sound.
It comes down to what you prefer, Visulization or Performance.
The comuter version is alot more advanced with many more modding capabilities and if your lucky enough to run it of the most advanced graphics card around (8800GTX) and on a 22" screen or higher then you are defenetly in for an experience of a life time. With the capabilites to add expansions and even make your own world the PC version is great.
However the 360 is very user friendly with easy controls. The game was built to run on the machine so your experience will defenetly be above average. But the 360 is not for an advanced user i would call it a kid comp, with no real depth in the game you could find yourself getting bored of the content pretty fast. Unless you can affored downloadable content.
In the end if you have a high performance comp with a main stream graphics card then the PC version is defenetly for you. If not jump onto a 360 and start leveling otherwise your going to go made overlocking your graphics card and codeing text* files.