I've never ridden an Allez, but I rode a lot of Treks (even an expensive Madone), and I never liked the ride. The Madone 5.2 was getting close, but then we were already way out of my price range. It came down to a choice between a Roubaix and a Tarmac. Since I was a new rider, I decided to go with the less stretched out feel of the Roubaix. Since then, I've dropped the stem significantly, and I wish I had the Tarmac also. I just really liked the feel of the Specialized bikes and the quality of components for the price point. Since I got my bike, I've noticed that the same price levels have gotten worse in the components department, so maybe if you're looking at new bikes, they'll be more comparable in that respect.

They're all good bikes. Ride them lots and you'll have a better idea about what's best for you.