Both have their strengths and weaknesses. Like you mentioned, Rayquaza does have better stats than Celebi, and (at least in my opinion) has better moves available than Celebi. However, with a Hydreigon already on your team, you would have two Dragon Pokemon, making your team very susceptible to Ice-type. Celebi is weaker with (again, my opinion) a weaker available moveset, and more weaknesses than Rayquaza. All said, I would choose Rayquaza, and if really needed you can switch out your Hydreigon, but it probably won't be necessary. I recommend switching out Outrage for a different move, such as Dragon Claw, Dragon Pulse, or maybe Thunderbolt, and swap out Overheat for Flamethrower. Even though Outrage and Overheat are powerful, the confusion and stat loss respectively associated with each move can hamper Rayquaza badly. Keep Fly and Stone Edge, or swap Stone Edge for Dragon Dance (Tutor move).
I hope this might have helped in some way. ;)