I've got an 07 Roubaix Expert and am in the process of purchasing an 08 Tarmac Expert. Both of these bikes have the 8r Az1 frames so comparing them is like comparing apples to apples. They both employ the same construction, but the Roubaix adds in the Zertz. I've got over 8000km's on my current Roubaix and when I got on the Tarmac I thought 'Wow, this feels just as supple (vertically compliant) as my Roubaix!'. The test ride wasn't long enough to really notice the difference between the two in the handling department, but my wife who has the same bikes did notice a difference in handling and acceleration. I plan on using the Tarmac for the shorter rides and leave the Roubaix for the longer hilly rides since the Roubaix is a compact and the Tarmac standard. Who know, I might just end up riding the Tarmac most times if it's just as comfortable and more responsive.
Either way you will be getting a good bike, but if you intend to race you may want the Tarmac over the Roubaix. I originally went for the Roubaix due to the comfort level and the fact I will not be racing, but now I'm looking for something a bit more aggressive and responsive. Really, the Roubaix is a great bike and has served me well. It's been a blast to ride and has gone as fast as I've been able to push it.