The reason Messi needs to prove himself at another club isnt anything to do with him- it's simply down to Barcelona.
There's something about Barcelona that suits a certain type of player due to their 'DNA' that is felt in their football. It's the same reason why a more individualistic footballer doesnt tend to do so well at Barcelona in comparison to a team oriented one.
I'd have like to see these team oriented players play in a team that isnt so team oriented over the past 2 decades to see how they play and achieve their success.
C Ronaldo is completely different to me. He didnt play in weak teams but he played in different teams with different players - he is important for a CL alongside Rooney, Tevez, Giggs and Scholes, he is important for another with Benzema, Bale, Modric, Isco, he is important for an Euro alongside Renato Sanches, William Carvalho,Nani & Eder.
He is selfish and one of the most individualistic players I have ever seen but where a player like Zlatan might fail due to their ego - Ronaldo will compete in different teams and territories and aim to come out successful.
Messi is the most skilful player I have seen. However Football remains a more a competitive sport to me than a type of entertainment. Thats why C Ronaldo is the greater player to me - he has proved to me that I can put him in any team, any league and any scenario and he will most likely come out successful.