What is the difference between these two bikes?

Geometry. If you look at the specs, I believe everything is the same. My LBS has the SuperSix for $2000 flat. The Supersix has racing geo while the Synapse is more for cruising. The SuperSix also looks sexier - which automatically makes it the better buy

Really? I compared them side to side and I find the Synapse Carbon 105 more attractive than the SuperSix.

You can also add/remove stem spacers on the Synapse if you are looking for a more aero position. The Synapse is a capable racer in my opinion.

105 is more than adequate for a new rider or a racer.

I had 0 issues with my 105 gear when racing and training on it. I later got Dura Ace because I could afford it and got a great deal on a used setup, but noticed little, if any, performance improvement.

Rode the Synapse Carbon 6 Apex today for comparision to the Specialized Roubaix Elite. For me, the Roubaix is the better ride. There's nothing wrong with the Synapse, but over most of today's ride it was not significantly better than my Trek 1500. Funny thing is, I was thinking along similar lines for 80% of the Roubaix ride, but when I got off the Roubaix I felt as if I could ride another couple of hours. My butt was sore from riding on a too narrow saddle, but the lower back felt good and strong (this is one area where the Trek falls short). I thought the two would be much closer, since they are both carbon. LOL. Lends some credence to the claim that it's not all about the frame material. I was hoping the Synapse would perform equally, mainly because it's $200 cheaper than the Roubaix! The 2010 Synapse Carbon with Ultegre was Bicycling magazine's "plushest ride" winner. Hmmmmm. The Synapse has a shorter wheelbase - identical to my Trek - which I suppose could have some effect on differences in ride quality. The Synapse felt quick, but so did the Roubaix, and I think that's what surprised me about the Roubaix - comfy and quick.

Very interesting. Thanks for the report.

Did you get it? I seem to have now set my sights on a 2011 Carbon 6 Apex as my first high quality road bike. Anyone else got any input on the 11 Apex equiped 6?

Not yet, but looking to get it as soon as the snow melts here.

Capable yes.....

But the frame geometry (lowering the stem wont fix the issue) make it a tad sketchy for crits and high speed cornering.