I think I'd personally go for the EOS 40D. It has a faster frame rate for action shooting, it has an ISO 3200 setting for low light work, it has slighly lower image noise - plus it uses the same CF memory cards that your XT does. The added size and weight and the "smoother" operation give it a more professional feel (though professional feel can't be seen in the images!).
The XSi is also a very good camera, expecially if you want something smaller, lighter and cheaper. If that's what you need than I'd have no trouble at all recommending it.
I compared the XTi, XSi and EOS 40D in this article:
http://www.bobatkins.com/photography/digital/canon_eos_xsi_vs_xti_vs_40D.htmlSince the
EOS 40D selling for $940 (after $200 instant reabate) and the
Digital Rebel XSi is selling for around $760, I think the EOS 40D is the better deal.
If you also need a lens, the
40D kit with the 28-135IS and the
XSi kit with the 18-55IS both offer pretty good savings on the lens too.