I probably would look into other brands besides Trek, and also hunt around for what you might find as a leftover '10 in your size with Ultegra 6700. A couple things to note:

The 2.1 has the new 5700 105, I would lean this route
If you look at any leftovers, 105 has been updated for 2011, so unless it's an amazing deal, be aware that 2010 and earlier (5600) 105 may be worth a bit less
While Tiagra appears to remain 9-speed for 2011, i'm not sure how much longer this will be the case. I wouldn't be surprised if Tiagra becomes 10-speed in 2012 (using the old pull ratios of the breaks and FD), pretty much i'd guess that 5600 105 will become "Tiagra" next year. With a 10-speed setup such as 105 (or above), you can also swap parts/upgrade relatively easily, with 9-speed you're less likely to be able to upgrade without running into compatibility issues.

There's nothing wrong with the Trek, but there's nothing all that special about it either.

In short, if you must have a Trek (I believe there are better quality and better value bikes out there), i'd get the 2.1 just because it's 5700 105 which is brand new, and will most likely stay current for a few years. Either one (Tiagra or 105) will be fine for Centuries, and either frame will be fine. I would suggest riding as many bikes as you can and getting the one that's most comfortable to you. Higher end components usually get your lighter weight, or bragging rights Comfort is what will help you get through the Century (Comfort being a quality bike that fits you). How flexible are you? If you're a new rider, you may also want to check out bikes with taller head-tubes (slightly more upright position) such as the Specialized Secteur, Giant Defy, etc.

You may also want to search around for bikes using SRAM's new Apex group. It should be priced similarly to 105, but is lighter and can be configured with wider gearing. There's no downside to trying out various bikes whihc are equipped differently, except for a small investment of your time.