I would also say go with what feels better in your hand, but in terms of features and performance, the D90 blows the T1i out of the water. Not really fair to compare the two. A closer comparison would be the T1i and the D5000. The D90 is a step up.
Oh, and if you are seeing a D90 for the same price as the T1i, you might have found a great deal or you may have found one that is too good to be true. Make sure it is from a credible source.