Messi and Ronaldo are clearly the two defining players of the last 10 years, though for me what separates them is that Messi has continued to able to thrive as essentially a False 9 player in the Barca system whereas Ronaldo has become more of an out and out striker who requires teammates to put him in positions to slam the door. If you look at the way they impact the games, it really comes down to a matter of taste: Messi is someone who weaves a move together, Ronaldo is a hammer to a nail.
A lot comes down to personality too: the preening diva versus the quiet enigma, one a physical specimen, the other a bit undersized.
Trying to parse their abilities via their international duties is very difficult too because there is no control to make the comparison even. Argentina have had A LOT of coaching turmoil which in turn seems to always turn up on the field and they never seem to turn out their best starting XI. Oftentimes, they advance through a sheer moment of magic from Messi. The fact that they don't just win everything as they seem to do in Barca is just as unfair as with Portugal and the teams Ronaldo has been a part of. International soccer is such a weird competition in general, with so many random outcomes, especially in continental play. Remember when Greece won the Euros?
Take care all.
