I have direct experience of the two models that you are looking at (I have a 450 /XSi, the previous model, which I am very happy with). On paper they are very similar; the main differences are improved meter, increased resolution and full video capability on the T2i/550 . However, the meter and resolution on the T1i are already very good so unless you really want these things I suggest that you use the money that you "save" for a second lens.

There is a good comparison of the two cameras here: http://www.dpreview.com/previews/CanonEOS550D/.

Note: EF-S lenses are cheaper than equivalent models in EF range. The downside is that they cannot be used on Canon models (e.g. 1 ) with full 35mm sensors. If you think that you might upgrade in future buy EF lenses if you can. However, as a beginner on a budget, two lens are better than one because you can learn more that way. If you only have one lens, go for something like 28-135mm.