I would say it depends on many other things you haven’t mentioned.
Rings can get pretty stale if that’s the only place you have to ride.
Doesn’t matter how many jumps you have if the person who is responsible for setting them doesn’t know how to to it.
A gymnastic can be more constructive than 8 pretty jumps set for inside/outside/outside/inside etc.
Good footing is good footing–whether it’s in a ring or on grass. Rings can be deep or hard, and need to be maintained regularly to avoid having a rut around the rim and holes on either side of the jumps. Grass needs to be mowed, and ideally, provided with nutrients and topdressed with some sort of loam/sand.
Grass can be delicate in wet weather–there are times of year when you really can’t use it much without damaging it. And if you are doing a lot of riding (lots of horses), it will not stand up to it without regularly shifting where your work area is located.
We don’t have a ring. Hasn’t stopped us from being to accomplish what we want to. We can and do ride on our grass, or ride out, and there are rings nearby we can use if we need to.
I think it’s more largely a matter of preference and the quality of the maintenance involved than anything else.