It's a great problem to have!
They are both as capable off road in the right hands. I have an MY12 D4 and love driving it. I frequently use the 3rd row and can't see me fitting all my camping gear in the RRS.
Today I had to pick up a chaise style lounge suit that I bought off gumtree. The girl I bought it off said no way it was going to fit. It swallowed the whole thing no trouble.

