As a Roubaix owner I'll say that Z5 is priced very well. My last bike, a carbon fiber Giant OCR C3 w/ a partial 105 group, listed for $1800 in 2005! The Roubaix's are all SL4 this year which up until last year you couldn't get for less than $3800. The 105 version has a lesser grade of carbon (8r vs. 10r.) The Roubaix looks like it also has 105 brakes, a slightly better FSA crank and Fulcrum wheelset. I would test ride the bikes you're interested in and make sure the tire pressure is the same on all of them.