Actually, the 2.1 Pilot is not a full carbon bike; you have to go up to the Pilot 5.0 for that. The 2.1 is made of Treks' "ZR 9000 [aluminum] alloy" and has carbon seat stays and fork. In looking at Trek's website, I can't really tell you what benefit the ZR 9000 alloy has over Trek's Alpha Aluminum, i.e., the frame material used for the 1600. You may have to ride both of them to see if there's a difference in their "feel."

Given that the 1600 and 2.1 have comparable components, I would buy the one that "feels" and "fits" the best. The Pilot has more of an upright, relaxed geometry, which a lot of recreational riders find more comfortable. I think most bikes, however, can be set up to provide a more upright geometry. I agree, too with KSH that you shouldn't buy a bike based on the saddle. They're easy enough to change out.