Just because Messi scores against PL teams doesn't mean he wouldn't struggle in the PL.
We bring this up not because the PL is harder but because Messi will be playing in a team outside his comfort zone.
Pep's City would be the only club he could play atleast to 85% of the level he does for Barcelona without ever having the likes of Iniesta, Xavi, Neymar, Suarez, Busquets, Puyok and Pique. Sure he would have Fernandinho, Sterling, Silva, De bryune - but no way would it be the same level of consistency he does at his home town club that ever since Pep - was built to get the best of Messi.
Ronaldo at United wasn't abnormally made to be the main attacking threat at United - he was given equal importance to Rooney & Tevez with usually Ronaldo outdoing them. At Madrid he got more targeted by their team as the main attacking threat but as a LW + he would find himself usually in the same position all the time whilst Benzema & Bale would play on the other side.
All Messi is - is more eye catching to watch. Ronaldo has better statistics and a CV across the world.
Whilst Messi plays - people will always talk about him being the GOAT for his wondegoals against betis and getafe he pulls three times a season - but when both retire - people will open their eyes about Ronaldo - seeing the things he has won, for the different teams, for the country whilst scoring the most goals and even assists.