I'd email Felt and ask about the diff between the two. As for endurance having a short TT and taller head tube, that's for comfort--makes the rider more upright and thus less weight on arms and less hyper-extended neck. Same as a shorter stem and/or more spacers. Have you had a pro bike fit done? Even if on an existing bike, a good fitter can make recs for it or what to get on your new one. Getting a fit done changed my cycling life. Hope this helps.