Against small teams like Alaves, Getafe, Saussolu, Empoli I would prefer Messi. He loves to score against them.
Against big teams of course Ronaldo. The bigger the team the stronger Ronaldo became. I am 1000% we wouldnt get past against Ateltico last year with messi instead Ronaldo, maybe we would have won 1-0 or something. The last round Messi turned himself was 2013 against Milan.