Both footballers, both athlete's, only one of them was on growth hormone as a kid to be stronger and faster than anyone else his size(and most people much bigger than him)
Seriously though, both excellent players, Ronaldo tips it for me as he's faster and stronger than Messi. They are generally impossible to directly compare as they play in different teams, for all the crooning over Real's "best squad" rubbish, a lot of that team is poor, defensively there isn't a truly great defender amongst them(Varane really looks something special though, but has barely played as of yet), Ozil, Di Maria, Kaka, Modric, Khedira, Benzema, Higuain, to call them anything other than massively inconsistent is to simply be lying. All can have great games, none of them have great games very often, even when they are good they aren't really close to Fabregas, Xavi, Iniesta, Villa, Messi or Ronaldo(the last two are ahead of the other lot but they are all miles ahead of the Real Madrid lot).
Ultimately Barca would be good if Messi was injured, Real would struggle very very badly, with different quality players around you, with a different manager, and with different length of time with said players, comparing who is better is not possible. Messi has played with Fabregas, Iniesta, Xavi, for over a decade now, Ronaldo has been at Real for 3-4 years, and the team hasn't been mostly the same in that time either.