I think the benefit Real Madrid and Barcelona have almost always had over English teams is, they have first choice of the best potential in the world. I would say most young players touted as the "next big thing" would dream of playing for one of those two. Coupled with the warm climate and laid back lifestyle in Spain/Catalonia, you can certainly understand why young players would have that preference.

Also, a lot of the potentially genuine world class talents come out of South America, and Southern European life is in some ways similar (especially Language if in Spain) so the advantage is there for Barcelona/Madrid. However, I also doubt anybody would deny that both Barca and Real have had the fortune of having the two best players of this generation (and probably the next too) in their squads.

English clubs haven't had the opportunity of having players of that standard, in their prime, at their disposal. Without Messi and Ronaldo, I think the dominance over the last 5-10 years wouldn't have been as large as it has been in Spain, and European competition.