Megapixel difference is meaningless. As for the Xsi verus the t1i, one has video recording, the other doesn't. Other than that they'll both take good pictures should you have the right lens on it.
The T1i seems to do better at high ISO (anything above 800 ISO) which might be useful to you. I personally find a lot of uses for high ISO. The XSi only goes to ISO 1600 too, which to me would be very limiting. The T1i hits 12800.
IQ wise I've read they are pretty much identical. The T1i does have a nicer LCD monitor on the back, and to me this is quite useful.
Given the higher ISO performance and better LCD display, I would take the T1i for only $100 more. But as others noted, if this is at the end of your budget you might want to put the $100 savings for the XSi towards better glass.
I have an XSI and I would not upgrade to a T1i.
However, if I were purchasing for the first time, I'd probably get the T1i for a $100 difference. I could care less about that video function, but the higher ISO is something I miss on my XSI.
Totally agree with Bigtwinky here, as another 450D shooter. I wouldn't upgrade, but can see the use of the higher ISOs of the 500D. (Regardless, I fully intend to go full-frame with my next camera a couple years from now; the 450D is solid and imagine I'll still have plenty of uses for it even then.)