Assuming the seat height can be set up correctly on both frames and the reach can be set to a comfortable length then the real difference as i see it will be the head tube length. The HT length will be smaller on smaller frames resulting in lower bars or the need for spacers.

Having said that I havent looked at the geometry for either bike and I think the roubaix has a higher front end anyway for the same size frame. In that case it may not turn out to much of an issue.

how much seat post will be showing on the smaller frame?