I would try to decide on your budget first then look at the bikes available at your price range (as each manufcturer is sure to have something for that level). After that you will most probably find that most/if not all bike will be similarly specced so it will boil down to the test rides and ofcourse general feell for the bike.
I would say at $3500 (your quoted price dor the six13) you will upgrade a bit more in the trek line (if you can wait a few months maybe get the ultegra 10) and similarly there are just so many good brands out there.