The Specialized Roubaix Pro tires on the Expert are pretty decent tires. I've ridden both and don't think there's much difference in ride quality between the two. But I would bet the GP4000S (II) is at least a 2-3w better in rolling resistance. I've been really impressed with the GP4000S tires for longevity and puncture resistance. My rear GP4000S has almost 6500 miles on it and the wear dots haven't completely worn flush yet.