you guys are settling an argument for me. I say CB for these reasons.
You need to be able to backpedal well off the line and then fluidly turn your hips to get into position if the route is going past you, or break quickly forward if it's in front of you. You're trying tos tay with a guy who knows exactly where he's going, while you need to adjust to his movements on the fly. You need to be fast enough to cover receivers, but strong enough (unless you're Deion) to bring receivers down and aid in stopping the running game.
You are covering guys ranging from steve smith to Fitz, to Moss, to TO...short shifty guys, Strong Fast guys, long strides, short strides..are they gonna be physical are they selling one route to do another, and a CB has to respond and still be there.....And they are in an all out sprint dang near every play.
and you are on an Island.