I would start to have them both 6 before taking any decision , i dont know about your server but in mine only top25+ still use riley and they loose many matchup.
We grew up with all the rileys to be on top and now she seems to perform only in a attack dmg aoe comp even if this team can be countered by many things.
In crucible she is always a must for me as a tank but i never use her anymore in arena, only as 3rd team in epic(usually the weakest ).
What level are you ? After few weeks of all 90 you will see that she is not a must anymore for arena purpose.