My wife has a 2005 Sirrus and my dad has a 2005 Trek 7300FX (pretty much the same as the 7.3 FX). Both bikes have the same triple crankset (48/38/28) but the Sirrus came with a road cassette (12/25) whereas the Trek came with a MTB cassette (11/32). The Sirrus thus has slightly higher lowend gearing but closer spacing between those gears. Because of the 11T rear cog, the Trek has high end gearing similar to many road bikes (a 48-11 combo is slightly higher than a 52-12).
The Trek frame is "beefier" than the Sirrus most notably due to the reinforcement near the headtube. The wheels are heavier duty on the Trek as well.
The Trek uses MTB style V-brakes whereas the Sirrus has mini-V's. Becuase of this, you can run wider tires than on the Sirrus (something to be considered if you want wide tires and fenders).
Basically, the Sirrus is closer to a road bike and the Trek is closer to a MTB. The Sirrus costs (or it did when I was looking) significantly more as road bikes tend to do. Having ridden both (for short rides only), either would make a great commuter with the Sirrus having the slight advantage in my opinion only because I think the Trek is slightly over-built for a road-only bike.