So in your opinion would it be better to pay the extra price for the 20d, but keep it longer than the XT?
Lastly, to truly answer your question directly, yes it is better to pay extra for the 20D and keep it longer than the XT. Some might say to upgrade the body and keep the lenses. Why not keep both? If you love your 20D and treat it well, and take good care of it, it will serve you for many years. I've had my Canon SLR for more than a decade (closer to two) and it just failed me two years ago.