I do recall latinoheat (who has always struck me as the most knowledgeable and long-serving La Liga fan here) commenting that La Liga was basically 4 or 5 good clubs and then 15 clubs the standard of Sunderland or West Brom (do correct me if I'm mis-remembering, latinoheat).
Since a lot of Messi's hype derives from his phenomenal goalscoring stats, one simply has to look at the standard of opposition. Would he score 50 goals in a league season in England? Well, Shearer, Suarez and Ronaldo didn't (best: 31, also the joint record), Henry and Van Persie didn't (best: 30), Drogba didn't (29), Rooney didn't (best: 27), Van Nistelrooy didn't (best: 25)...I could go on all day with a list of all-time great strikers who didn't even get anywhere near the ludicrous statistic that is 50 goals achieved by Messi (and the 46 achieved by Ronaldo).
To view La Liga achievements through the same lens as Premier League achievements is naïve at best, dangerous at worst. I am not questioning the talents of the man, he is without doubt one of the best of our generation. But I will always be more impressed by Cristiano as I have seen him successfully and delightfully ply his trade in England. Messi will always have a question mark attached to him in my mind until he does that.