The best camera for the job is the one you actually decide to carry. I have lots of heavy photo gear (and lugged about 25 lbs. of it across Australia all last summer) and can tell you that there are times when lightest is best.
Translation: If you had an XTi and you are worried about the weight, don't get an XSi. I own an XSi. Yes it's lighter than an XTi, but not *that* much lighter.