I'm the same general size as you, and own a Giant OCR3 composite, basically the same bike as a Roubaix. The bike is perfect for my club rides, hideous climbs (who am I kidding, all climbs are hideous for Clydes), and commuting. It's fast, maneuverable and comfy.
I own another bike with racing geometry - a steel singlespeed with a narrow wheelbase, steep head and seat tubes, big handlebar drop - and it's fine for short rides, but gets really annoying for anything over 30 miles. It's super quick and turns on a dime, but it also beats me up and feels sketchy over rougher roads. That's close to the Cervelo you're looking at.
I'd say go for the Roubaix. The slightly slacker geometry and longer wheelbase will give you comfort, and will support the riding you say you want to do. The tall head tube will give you options - run spacers and a flipped-up stem for a century, flip it down after you get in better shape and can handle a more aggressively aero position.
I guess it's the difference between a fast BMW and an F1 racer. One's built for speed and comfort, the other is performance at the expense of comfort. As clydes, we have less tolerance for that sort of suffering over long distances. That stuff is just plain tougher on big guys.
So... what Tom Stormcrowe said.
Last edited by schnee; 11-12-07 at 01:35 AM.