I just switched from a Merlin Road Ti frame to a Specialized Roubaix (2007 model, the new ones are nicer) and I freaking love it love it love it love it. Is that over board? The head tube on the generation that I have is enormous, but I like it that way. I think the newer ones are more restrained, head tube wise. They're also just as stiff as a tarmac...
Anyway, people in the pro peloton ride this frame, so clearly it's not a "slow" bike just because it's "comfortable"
I think the Madone 6.2 is probably more comparable with a Tarmac (i.e. race geometry)
Here's an interesting back to back comparison of a Tarmac and a Roubaix:
http://redkiteprayer.com/?p=1408
My advice to you: ride them both at LEAST 30 miles and buy the one that makes you want to go riding every day. If you won't ride it, you won't like it. An Allez is basically a Al Tarmac of course, and if you're not fussy, I see last year's Tarmacs on sale all the time.