New bike this year and the six13 pro2 really cought my eye until... The pro frame only has a carbon downtube. Is there really a point to a carbon downtube? The r-5000 can be had for only $100 more and it has dura-ace vs ultegra. Will the downtube do much for the ride quality as a full carbon frame or does this fall along the lines of carbon seatstays where the prevaling opinion is they don't make a difference. The other bikes of interest are the tarmac and a tcr comp2 both full carbon.