Personally I think Ronaldo will last longer then Bale does. Bale is completely reliant on his pace, and power, while Ronaldo has much more to his game when you look at his ability on the ball. I think it's fair to say that Ronaldo has another 4-5 years left as a world class player, while Bale will probably start declining at the age of 30. There isn't another player like Ronaldo who looks after himself in the same way, which is why you can't compare him to others. Bale's 6-10 years as a top player if he came here, wouldn't be as good as Ronaldo's next 4-5 years for sure. I'd be willing to bet quite a bit on Ronaldo scoring more goals in those 5 years than Bale does in 10, such is the difference in their quality and goalscoring prowess.
Bale would undoubtedly help us with the league title, but he wouldn't put us at the very top alongside teams like Bayern IMO which Ronaldo would do easily. We would have a much better chance of winning a Champions league or 2 with Ronaldo in the next 5 years then we would if we got Bale, and in the end, what we really want is trophies, not which of the two players would last longer as a top player. Who cares about that when one of them will put us amongst favorites to win the champions league and premier league every year, and the other just make us stronger in the premier league but wouldn't make a huge difference with respect to the champions league. The opposition definitely wouldn't fear Bale as much as Ronaldo, and we would have to nullify Bale a little and put him on the wing because he would not be the best player at United, like Ronaldo would be if he came. I don't think for a second that we would put Bale behind the striker and turn him into our main player if he came, and he is half as effective on the wing goalscoring wise. Ronaldo on the other hand would slot in much easier and while a few others might have to work harder in defence, we would be much better off making sacrifices for Ronaldo which would improve any team in the world, while if we turned Bale into our main player, I don't think we'd be better off for it considering Van Persie is a better player.