Okay, I played with Soundtrack yesterday, and it is a great program. It is quite different from Reason, actually; it is for people who want a simple way to make music, though you may lack much musical ability. It comes with many, many pre-recorded tracks that usually loop very well, and you can add your own sounds if you feel that something is lacking.
Reason, however, lets you tinker with every little nuance of a song, tweaking the bass line to sound just right and modifying the drum and synth effects until it flows perfectly. You can use your own sounds in Reason as well, but the loops you might use are user-created typically instead of pre-fab. Reason does have presets, which you get at with any new file, but you can tweak those presets to your hearts desire as long as you know what you're doing (and even if you don't; in that case you'll pick it up as you go). Soundtrack is limited in that you can't really tweak any of the components of a song significantly.
Soundtrack has a much simpler interface, but it's a simpler program, and Reason's module-based interface is really cool. I don't know if you've discovered this or not, but hit Tab to flip everything around and see what's connected to what.