Wow.. what a toss up... I flew Envoy class back when it was on the 767's.. and it was nice.. flying out was good.. becasue we could watch a movie and sleep well.. but it was only 6 1/2 hours or so... coming back.. it was sooooolong... and we were also in Envoy class... but it made all the difference. I flew coach Madrid-PHL and Envoy Paris-PHL.. and I can tell you that Madrid flight felt like it was never going to end..
Depends on your preferences.... If you can sleep in coach and there is a light load.. take coach.. you will probably nap either way in Europe.. and the Envoy on the way home is a nice end...