I think a big reason why people call La Liga a 'two horse race' is because the clubs who spend the most are Real and Barca. For example, a hot prospect worth 60M wants to come to La Liga.... Villareal, Espanyol, Valencia and others alike are not going to spend that money, they simply can't. So the best players end up going to Real and Barca. Both clubs have stock piled players on some 'arms race' shyt.
In the Premiere League, you have more clubs that can spend that type of money so it looks like the league is really balanced. Man City, Chelsea, Liverpool, and United can come up with the cash and top players are spread out, so it feels like a more balanced league. Even though as someone pointed out, both leagues have had more or less the same number of league winners since 1992, La Liga feels more dominated by Real/Barca.
I think things will get hella interesting once Ronaldo and Messi retire/leave their teams.