Wow, I just went through this decision last month. I test rode both bikes extensively. Initially, I liked the Trek because it seemed to jump off the line snappier, but I found that the Tarmac seemed an all around better ride (and I loved the color scheme).
However, I returned the Tarmac two days ago and got a Roubaix Pro: the Tarmac had too aggressive of a riding position, and it was wearing me out in the neck and shoulders quickly (that being said, it was still more comfortable than the Trek). Plus, I decided that I just wanted better components, too.
I was comparing the Roubaix Pro to the Trek 5.5. I really wanted to get the 5.5, but it was more of the same with the aggressive riding position, and it hurt my neck and shoulders more than the Tarmac.
Between the 4.5 and the Tarmac, I prefer the Tarmac.
I did have all of the bikes that I tested sized to me: the LBS staff was very knowledgeable and went to great lengths to fit me to each bike.