I'm a big fan of stabilization because I do a lot of low light shooting of subjects that are stationary. I'm also a big fan of prime lenses (fixed focal length, not zoom) because I love tack-sharp images. Unfortunately, you can't really have the best of both worlds with Canon unless you're buying very expensive telephoto lenses (200mm and longer).
So if you think you'll be shooting moving subjects in low light situations, then prime lenses are the answer. However, if you want to shoot interiors, cathedrals, museums, etc. where your subjects will be stationary and tripod use will be will be forbidden or highly inconvenient, then IS will help immensely.
The great thing about prime lenses is that the sharpness is there for you regardless of light conditions. Primes are nearly always sharper than zooms. They're also faster (that is, they have wide maximum apertures, such as f/1.2 through to f/2.8), which gives you the ability to play around with shallow depth of field (sharp subject against a blurry background).
Pardon the parenthetical explanations of simple terms, but I gather that the OP is rather new to photography.