I have both the all aluminum and the aluminum / carbon seat stay version of the Nashbar frame, both in 58 cm. The all aluminum weighs 3.66 lbs, with cable guide, RD hanger, and cage bolts installed. The alum / cabon weighs 3.44 lbs in the exact same condition. I just built the alum / carbon stay version, and have less than 100 miles on it, but I can say it does ride more smoothly than the all aluminum, with no perceptible loss of stiffness. I didn't think the all aluminum was harsh, and routinely put 75 to 100 miles on it, but it does seem that the carbon rear end transmits less "buzz" from the road surface.
They are short for their height, and the paint chips very easily on both. I got the all alum for $80 + shipping, and the alum / carbon for $120 + shipping. Play the sales with them.