I believe Roues Artisanales did an aero test a while back. The Xentis Mark 1 TT ended second behind the Zipp 808. I am pretty sure the Enve wheels were not included.
Well Enve shows their data on the 6.7s to rival to Firecrest 808s which in turn are better than the 808s from that test. Sounds to me like the 6.7s would be more aeroynamic than the Xentis and will handle better in crosswinds as well. They are both fast wheels, but with the improved stability of the modern rim shapes I wouldn't keep the Xentis over the 6.7s.
Ah Enve, the darling of commercial wheelbuilders...
Now I wonder why.....?
OP - why dont you go out and time your next 20 loops of a familiar test ride and log the data and see which wheel is best for yourself?
Chances are, if you have to ask on a forum which wheels is faster, that you are not fast enough for the aerodynamics of either to matter.
If you are fast enough you will have more data on speed and handling impressions by giving them a spin.
As a rule of thumb - people tend to like what they already own, and professional wheelbuilders tend to like what they sell, so it is no surprise to hear that they like Enve.