What does speak for the strength of the Spanish league is how well their teams do in the Europa League/UEFA Cup,
English teams are obsessed with fourth place making the Europa League a distraction when it should be a priority. If the Europa League guaranteed a champions league place you wouldn't see the same results.
Also it does make perfect sense to compare Ronaldo and Gomez because one can outscore the other in the champions league quite comfortably yet can't match him in those seasons in league competition. That tells me that one league is a lot easier to play in than the other. You can talk about easy draws but there is no match in the champions league that compares to the mediocrity present in the bottom half of La Liga.
Gomez and Van Persie would comfortably score 30-35 goals in La Liga. I have no doubt about that whatsoever.