Since it's mostly flat, you should probably consider the SU100 -- you don't really need the super-low gearing of a mountain bike given the terrain you'll be riding. Same goes with the suspension fork.

If brands other than Trek are an option, you might also consider the Kona Dew ($400). It's similar to the SU100 (both have aluminum frames and 48/38/28 cranksets) but it has roadbike wheels (700c) rather than mountain-bike wheels (26").

If you prefer 26" wheels, then the SU100 seems like a pretty good option. I like the silver finish too.