Remember that you're comparing an entry level dSLR with the best AF performance (Canon T1i) with an entry level dSLR with close to the worst AF performance (Nikon D5000), so there will be some difference, and it's not necessarily related to the lens.
Also keep in mind that Canon's XSi is likely to be discontinued soon, and will probably be replaced with a newer model. If you've got some time before you need to make your purchasing decision, you might want to wait for that.
But if you wait too long, you may end up missing some shots you really wnat, because you're still learning to use the camera. So don't wait too long.
- The lens is the thing.
- 'Full Frame' is the new 'Medium Format'.
- "One good test is worth a thousand expert opinions." - Tex Johnston, Boeing 707 test pilot.