I'm closing in on buying my first bike as an adult, and am working on a $300-550 budget. I'm looking for something that will handle the urban streets (potholes, curbs, stop-and-go traffic, etc) as well as at least some light trails. Because it's more or less my first time riding in a while, and my first time ever in an urban environment, I want something that's going to offer stability (while still allowing me to get good exercise, do the occasional commute, etc).

I've heard that some people actually go with Mountain Bikes, while others strongly recommend the hybrid. At this point I'm narrowed to the Trek 3900/4300, the Trek 7.2 FX/7.3 FX, and the Specialized Sirrus.

Any thoughts on the best way to spend my money...and whether it's worth spending the extra money on the higher-grade models (4300/7.3/Sirrus) vs the lower ones (3900/7.2)?