I just did a long century (120 miles) on a Roubaix elite and had a great time.
If that means anything to you, there were several articles about the Roubaix Comp in the June or July issue of Bicycling magazine, they pretty much echoed what your salesman told you.. speed and comfort.
The 2004 elite is not full carbon, so I cannot even imagine how comfy the 2004 Comp is...
Before you buy, check out the 2005 Roubaix... they look even better....