E Series all the way. I've rowed all 3 and own an E. If you're mostly rowing / paddle guiding from the back, the square stern is super nice. The extra rocker with the pointed bow pierces thru waves and holes much better than the D and especially the R. Almost every commercial raft you see on the Upper Animas is an E series and for good reason.
It works fine as an oar rig too, I've taken mine on tons of 7+ day trips. If you are really stuck on the D or the R, I wouldn't even consider the R.