Consider a Felt F75 as well. It has carbon seat stays and 105 components like the Trek but is several hundred dollars less.

I think getting 105 components or better is a good idea since you know you're into biking & that way you won't feel the tinge of regret that will make you need to buy a better bike in a year. IMO, the 2.3 or Madone 4.5 would both serve you well for many years.

If you decide to go with the alu/carbon frame combo, do check out Felt as their comparably equipped bikes are less money than the Treks.