It depends on how complex you think your projects will be. Are you a film student? Is it just a hobby that you indulge in when you have free time?
Studio is a software bundle that includes extra software to bolster Final Cut by offering things like color grading (Color), soundtrack design (Soundtrack Pro), DVD authoring software (DVD Studio Pro), motion graphics (Motion), etc. If you're involved in any kind of media production program in school then I'd definitely go for that as it would be very valuable to you.
If you're in the amateur realm of filmmaking and just want to use it for the occasional video for class or for fun, then Final Cut Express would be better suited for you as it offers all the functions of Final Cut as far as editing goes, without all the extra frills that honestly you probably wouldn't need.
Ultimately it's your call, check them both out on Apple's websites, watch some tutorial videos and whatnot that they have on there so you can see what everything does. A bonus would be if you know someone or someplace that has either or both of the software packages so you could check them out first-hand.
Good luck with your decision!