I wouldnt r2 them yet. Gulk is good, but depends on how many r5 you have.
Both of these champions have niche utility. Not sure I would rank either of them up to r2, especially as my first six star to that level. Sooner or later you will get a terrific champ and you will be happy you have the resources to rank them up. If forced to choose I would say rhulk, especially if you have a high level capiw to go with him.