A R&R romance is unbelievable. They had their true last opportunity at the end of S4, since then, the baby and marriage in Las Vegas were good ideas, but not result of love in the way they loved each other the first 4 seasons. Leave the past and look for something new. R&R ending up together wouldn't be believable IMO, and I would think they do it because of Emma, as in the end of season 8 where they seemed to want to force themselves to get together again.
Instead, J&R is much fresher, and although we know they wouldn't last forever, since their chemistry has an expiration date, until then, we can have an interesting highschool style romance. Unfortunately we know they won't end up together because of the Joey spinoff, but I hope they give some time before they make them break. So I hope they let them at least half the season (9 episodes) to explore what they can have, and after that, really who wants to go backwards to the same R&R, just because it was the main storyline at the beginning.