I have a Rebel XT and was at this same decision last year. I spent the extra $$ and got the Canon 100mm f2.8. If you have the $$, I highly recommend it!
I asked this same question in several different forums before I bought mine. Nothing but 100% positive feedback, and I have yet to hear (or read) a single regret from an owner after they've bought one. There aren't too many lenses in that category....
Concur with AZTBS though, make sure you have a tripod when shooting all macros.