Did you make sure both bikes had the same pressure in the tires (tires were inflated to the same amount)? That will make a big difference in ride quality. I just mention it because there have been a number of people who have ridden Allez's with and without the carbon seatstays and they said they couldn't tell the difference. That was between a $1200 and a $1400 model or something, though.

Like other people said, the Sectuer and the Tarmac are in completely different price ranges. It's the Sectuer / Allez that are aluminum and are cheaper (endurance road / race road), it's the Roubaix / Tarmac that are full carbon and at the same price level (endurance road / race road). You'll notice vibration dampening more and more the longer you ride (like you'll notice it more after 20 miles than after 10 miles). But on the Rouabix / Tarmac level, plenty of people ride Tarmacs on centuries. It's a little different because a high end tarmac has a lot of vibration dampening that they can put in carbon but probably not aluminum, but if you want to see a debate on the geometry differences, here's a recent thread that might be interesting -

"Any Specialized Roubaix owners who've "switched" to a Tamarac (or other "racier" bike"
http://www.bikeforums.net/showthread...0#post11275270

For people who's only goal is to do a century a more relaxed geometry is better, but for people who ride all the time and build up the muscles for the more aggressive positioning and steering of the "race" bikes, it doesn't matter as much.