Here is a post I made when comparing the D90 and the T1i, I didn't want to have to re-type it all LOL
"I have owned both of these fine machines but i like the canon one better. The high iso at 3200 and 6400 are better than the Nikon D90 and the video quality is nicer at 720p. The D90 did not even have 1080 as an option. This second option looks nice on the Canon, as long as you hold relatively still and your subject does not move too much (20 fps :/) I used the T1i with my 70 - 200 L and 17 - 40 L and a 50 1.4 on its first real outing and it was wonderful. I ended up returning the D90 a few months ago because I had spent something like 2000 dollars to get it and its kit lens 18 -105 VR, the 70 - 300 VR and the 50 1.8. By returning it I was able to exchange it for L glass for my 30D at the time, and it was much better even without the video the D90 had. Now that I have sweet video on the Canon side I bought the T1i and I am so happy that I did not end up going full on with the D90. I think this screen is wonderful and the live view shooting helps a lot more than I thought it could when I did not have the option on my 30D.
In conclusion I think you should go T1i because you can get very nice quality videos, very nice high iso performance, and pretty good battery life. I took something like 380 pictures today at the zoo, alot of them using live view, and about 20 - 30 minutes of video and the battery still showed half full. I also spent a lot of time reviewing those photos on the great screen. If you want more battery life, buy a second battery :P Buy lots of memory cards though, I went through 8 GB today to the last 5 mb jpeg, after I stopped taking RAW to get more room."