I recommend the 5.1, when i bought my 07 trek 1500, I was torn between that bike for $800, or a caad9 for around $1200 (I really wanted the caad 9) I went the cheap route, and guess what happened? I regretted it and I just put in an order for a caad 9 frame.
on the other hand, I test rode a 56cm systemsix to make sure my caad 9 team frame (BB30! shhh.) would fit (no 56 caad9's in stock). guess what... the half-carbon bike with ultegra and carbon Si cranks didn't do anything magical. the road vibrations were softer, it handled better, and it was way stiffer, the ultegra was slightly faster and smoother than my 9spd 105, and i thought it was the coolest thing in the world. however, when i hopped back on the $800 trek, i realized that a bike is a bike and going with the 5.1 wont make you any faster.
so there are two sides of the story. I think the 5.1 is worth it because you will probably appreciate the extra stiffness and you wont have the "what if" factor in mind. If i were you, I would get the 5.1, no regrets!