I loved my XSi, IQ was nearly indistinguishable to the 5d. I think the video would be the most distinguishing factor to me. You'll enjoy either camera though, I don't think you'll be unhappy whichever you choose. I normally wouldn't care about video, but you have a fun lens for it. I guess I can't really offer a suggestion lol.
If your not interested in video I think the xsi is a really good deal. I remember seeing a deal a few days ago for an xsi, 18-55is, and 55-250is for ~$650. The lenses are worth ~$300 so it makes for a great inexpensive cam.
I have an XSi and am amazed with its IQ but it's a step below 5D, esp. regarding shadows in high ISO. But, for a cropper, it's the second best camera ever IMO, only beat by 40D.
About T1i, the only *cool* thing on it is the hi-res screen. HD video @ 20fps and mono? No thanks
I also have the 5D and the xsi. Share the same with the above posters about image quality. This little camera will amaze you when printing 8x12 prints. I cant see the difference with these cameras at this size. The kit lens is the big shocker also. I got the vertical grip also. For random shooting and personal use my 5D stays in its bag. The light weight and nimbleness of the XSI makes fast shooting a dream. You will find yourself going to this little gem as your go to...Highly Recommended..Bruce
I started with an XSi + kit lens! I can still get really good images from it. (Search for my images in the People Forum here) The IQ is pretty good, but now that I have the 5D2, I can tell a difference in image quality. I may be nip picking, so...
XSi's iso max out at 1600, while T1i can go higher, so it depends on if you really need high iso also... My XSi is also gripped, very nice! I have printed up to 30 x 20 on it, the image was pretty crisp and nice. It's my second/backup camera at wedding shoots now.
