You might want to consider this: you will never change the stays, but you can upgrade your components (altho you may never do that either). On the other hand, if the stays are shaped--and I assume they are, with that hourglass curve--then you'll get a little more forgiving ride, right there. Also, I wouldn't want to go below the level of 105. The 105 group nowadays is very good. Btw, are the shifters 105s? I think that's more important than the derailleur (and a LOT more expensive to upgrade later).
All in all, I agree with the last post: it will come down to how these bikes feel to YOU. You've just gotta take the time to do a serious test ride, and try to get a sense of which one feels best. I own 2 bikes, and love the one that cost less, probably because it fits so well.
As for the carbon seatpost, one thing to think about is that you will not be able to mount a seatpost rack on it. If you like to ride w/o baggage part of the time but sometimes need to carry stuff, it's really nice to have a quick-release rack that just attaches to your seatpost (Topeak makes a great one, and a bag that slides on & off quickly & easily).