I was in the same boat as you, except the XSi was on the top of my budget, i was looking at olympus 4:3 kits, however after using the T1i, better to spend more and not have to buy a new camera in a year in my opinion! heres some numbers
-T1i is 15.1 mp to the XSi's 12mp
-T1i has a different processor, allows it to take many more JPEGS and RAW in continuous shooting compared to the XSi
-T1i has 720 and 1080p video, however only full 30 fps at 720 compared to no video capabilities, this was my selling point because i do actually use video but if it isn't anything important to have in a dSLR for you, or you have a separate camera then this shouldn't be a huge factor.
-T1i has ISO of up to 12800, as opposed to the XSi's 1600 i believe it is?
Either way you go, they're both great camera's, if you shoot in lower light ever, enjoy night shots, or need video which were my selling points, then the T1i is an awesome camera, if these aren't necessary for you then the extra bucks may be saved, or put towards some nice glass