The XS is 10.2MP, 3888x2592. The XSi is 12MP, 4,272 x 2,848. So cropped square, your full size images are 2592 vs 2848 pixels, or about 10% bigger. Won't make much difference.
I have never tried an XSi so can't comment about mechanics or durability comparisons.
8289 is a good low shutter count, but it is still high enough that if the user was not careful, the camera could have a lot of cosmetic wear. When I am buying an XS, I look at the rubber grip to see if it is worn. I only buy cameras with near perfect cosmetics.
Contact me for photographic equipment or visit my home page at: http://macrocoins.com