I levelled both to 90 after my tanks and spent a lot of time last expansion on DRG but honestly, if I could only play one, I'd pick RPR. DRG is still fun but it feels more methodical to me, perhaps because I'm more experienced and know the optimal rotations but RPR feels a little more fluid in what I'm doing. I mained GNB though for most of last expansion and RPR feels somewhat similar in terms of it's DPS playstyle. I say level both though and see for yourself.