The T2i should be somewhat better than the XSi, but not dramatically so for long-exposure night work. Based on the community answers, it offers slightly better low-light performance and a bit more dynamic range, plus other general upgrades like higher resolution, better LCD, faster operation, improved AF, higher max ISO, and video.

For pure image quality in long exposures, though, the gain is described as modest—likely visible overall, but not a huge leap. At the pixel level, noise may still not look much better if you inspect files very closely.

So if your main goal is noticeably cleaner star-trail or night images, the T2i is only a small step up from the XSi. If you also want the extra features, it may still be worth it. If your priority is a more significant improvement, the community consensus suggests stretching your budget or waiting for a higher-tier Canon body rather than expecting a major jump from XSi to T2i alone.