I purchased a 2010 Defy as my first "modern" road bike (I'd owned some older steel bikes as a teen, but stopped riding for a number of years and recently got back into the swing of things), and I have been pretty happy with it. It is a comfortable riding frame, and I have taken it on numerous 10mile commutes to work as well as a few 50 mile "pleasure rides". It does seems to be a little on the hefty side, but not so much that it feels like a boat anchor.
Having said this much, both bikes have comparable components (shimano tiagra, same gearing, different FSA cranks) so you'll likely feel little difference in the actual drivetrain. However, looking at the geometry tables, the Trek appears to be a hair more aggressive (shorter headtube, slightly more "aero"). I say look around for another Trek dealer and give the bike a go, or try out a different model with the same geometry and a different drivetrain. Unless you're super anal, or change brands you'll likely notice little difference in drivetrain components between 2300, Sora, and Tiagra from the saddle. SRAM controls are different, and the shifting feels a bit different too, but I can't really tell a difference in the lower end Shimano stuff from numerous test rides. Try something with a better or worse groupset, but same frame and check the reach, comfort, and handling of the bike. That should give you a pretty good idea on what you prefer.
Get whichever fits and enjoy the ride.