Its hard for me to see a difference between the ISO's from 200-3200 on the T2i and D90. I suspect you'll not see a significant/noticeable difference between the two. The T2i does give more megapixels, thus more cropping ability. Also, with all the aftermarket noise reducing software, the noise issue is *almost* moot.
As anyone will tell you, you want to invest in glass, not bodies.
Are you vested in glass with Canon? I'm assuming not since you are considering switching.
In my opinion, I like Nikon's menu system better, and I like Nikon's OCF system better. I also like the AF systems better. Those are three of the varying reasons why I went with Nikon over Canon.
A cropped sensor like the T2i or the D90 will give you better focal length because of the hyper focal length or whatever, I think with Canon its 1.5x and with Nikon its 1.6x or vise-versa. Great for wildlife that you can't get close to.
Also, check this link out
DP REVIEW: SIDE BY SIDE. I found it helpful when I was deciding by in December 2009 which camera brand to go with.