I'd go D90 but I admit to being a prejudiced Nikon owner with 5 lenses.
If you had several lenses I'd say to stay with Canon by either upgrading your lens supply or getting the 40D to go with the lenses you have.
Which Tamron do you have? Are you getting good pics with your xt?
I'm sure someone with more knowledge than I will jump it but in the meanwhile WELCOME to the Forums and hang around and share what you do!