Neither are terribly spectacular defensively - especially at this point in time. I think you have it though. These are just my opinions, but:
In their primes I would say Ronaldo was the better goal scorer, and better in the air. You could argue that he's also the more 'clutch' player as well, if you believe in such things.
Messi had better vision and 'Football IQ' if you will, who could take you on one on one or thread the pass and leave you behind. Still a good goal scorer but a notch below Ronaldo in that respect. I'd take Messi on free kicks over Ronaldo as well, though I know some may find that controversial.