The point isn't who's a better player now; it's who's a better signing right now. Which one would you rather take for your team.
Ronaldo's 30. No real signs of decline but I'd be surprised if we didn't see some soon. I also suspect that he'd score less goals in a team that isn't Madrid. And I think Bale would probably thrive in a team that isn't Madrid, and he's five years younger.
Ronaldo could probably play as a centre forward from age 32 onwards, extending his playing career, but 5-6 years of Bale as your centrepiece seems like a better option.