If this discussion is about who is better ON PAPER, then it's Ronaldo. Rivaldo is a very good player with outstanding skills, but Ronaldo is a great player with exceptional skills. However, if we're talking about who is better ON GRASS, it's Rivaldo -- because right now he can walk.
I hope Ronaldo comes back slowly in the middle of the season when he don't really need him -- and when he can play without the pressure of having to prove immediately that he's the best player of his generation.
Even though they play different posititions on the team, Ronaldo is much better at his position than is Rivaldo. When you put aside the injuries he's suffered, Ronaldo is by far the most gifted striker. He's got pretty good size, yet has the dribbling skills that you might expect to find on a smaller, quicker player.
Rivaldo obviously is very skilled, but INHO he isn't any better than Zidane. He doesn't even play a central role on his team, he plays at the left. I personally feel that Rivaldo doesn't have what it takes to take control of a game and win it for his team if they need it. He may be a better individual player than say a RuiCosta or a Boban, but these guys can take the team on their shoulders and run the show.
So I would say that Ronaldo is the greatest striker in the world, and has a lot more competition to compare him to. Rivaldo, who despite playing on the left is a playmaker, has a lot less competition as there aren't nearly as many playmakers as strikers.