I was up at PCF and rode the Corsa and the Giro that they have available.  They were both very nice, but I think that if I went with Bachetta I would go for dual 650s rather than a 26/20 config.  
 
A couple of questions for you Bachetta owners out there:
1. I'm not sure about the extra $$ for a Corsa over a Strada...is it worth it?
2. I'm 5'7", and trialed the large frame on the Corsa.  It wasn't too bad (could keep both feet flat when at a stop), but technically I'm supposed to be on a medium frame.  Should I try out a medium frame?
3. I had originally wanted to get an USS bike, as it seems that on longer rides the hand position would be more comfortable.  On my old Lightnings the hand position is very close to the chest, and I occasionally had numb fingers.  
    Although I wasn't used to the handlebars on the Bachettas, they were pretty comfortable while I was testing.  How do they do on long rides?
 
-DA

Go with the medium frame. On Bacchettas, the seat adjusts, not the boom, so you end up sitting further forward on a large frame, which puts you higher off the ground. The medium would be better for you. As to which bike to buy, I think you should buy the Carbon Aero. It's only a few thousand dollars more.  In the end, each of us has to decide for ourselves whether pricier, lighter bikes are worth it.
 
I like the Bacchetta handlebar setup, but that's another thing that's pretty personal.

As a new Corsa owner (about 3 weeks ago) and previous USS driver, here's my thoughts.  You're spending quite a bit of money on a bike ,whether Corsa or Strada.  Get one that fits, you'll be glad you did in the long run even if it means waiting a few weeks.  I was tempted by a used large frame Ti Aero but am glad I went with the new Corsa in a medium for less money.  I'm 5'10" and could likely get by fine with a large but I think the fit to the medium is just right.  Regarding arm position, to be honest I'd have to say the USS is a little more confortable, but that DOESN"T mean the Corsa is UNcomfortable.  The seats also not AS comfortable, but again I can't say it's uncomfortable.  The longest ride I've been on so far is just under 3 hours and neither my arms or hands have felt stressed.  This was one of my main concerns as it is yours, and fortunately I had a chance to ride a medium Aero for a couple of hours to prove to myself that the position was going to work before I made the plunge.  I also spent a week riding a medium Strada before I bought the Corsa, and would say the difference is not very great in most cases.  Only place I can tell a little difference is climbing hills where the 4 less pounds is noticeable.  Ultimately I wanted to get a quality bike and was willing to pay a little more to shave off a few pounds.  I could afford an Aero but didn't think the large costs difference between titanium and aluminum were reflected in performance and only saved another pound or so.  I also don't like tinkering, so wanted to get one that I wouldn't be replacing parts trying to upgrade all the time.  Really just have to try things out for a while to discover what works for you.
Good luck with whatever new bike you decide on.
Robert

You can compare specs at Bacchetta's site. The Corsa specs are here. The Strada specs are here. Click the Complete Specifications link on each page to see all the specs.