whats better xbox 360 or a custom built rig??

Analog sticks are not abysmal for FPS. That's exaggerating. Once you get used to them they're ok. But the PC gives more accuracy, so it's a bit better.

And you obviouslt don't really care about achievements because you do not own a 360.

 

Actually I do own a 360 and a PC, so I have first hand experience, and I can tell you, FPS with analog sticks compared to a proper mouse is worlds apart.

 

Well I'm assuming you're more used to the console you play with more, which is the PC. So you prefer the mouse because you're used to it. I however am okay with both.

 

They aren't equal. using a control stick for FPS is refreshingly different, but it's certainly inferior to a mouse. In any case, the answer's pretty definitive. You should only buy the xbox if that's all you can afford, in an ideal world you should have both.