It's an interesting point indeed, but I don't think the quality of La Liga is the main factor per se. What Messi is doing now is quite special, although ultimately irrelevant if he can't reproduce it against Milan...
games scored in/games played (home + away)
Messi
20/35 (10/16 + 10/19) = 57.4%
20/33 (9/16 + 11/17) = 60.6%
25/37 (16/18 + 9/19) = 67.6%
23/27 (10/13 + 13/14) * = 85.2% (!!)
Cristiano
19/29 (10/15 + 9/14) = 65.5%
18/34 (11/16 + 7/18) = 52.9%
27/38 (14/19 + 13/19) = 71.1%
13/25 (8/13 + 5/12) * = 52.0%
So basically what this represents is that Ronaldo is having statistically his poorest season in terms of scoring in as much league games as possible, only scoring in 13 of his 25 games so far. Messi meanwhile has just taken it to a whole new level, 85% of his league games he has now scored in, the previous best was Ronaldo last season with an already mind-blowing 71%. Also note that the away form usually has the biggest impact, Messi has only once failed to score away from home so far.
Cristiano is for the first time since he joined RM significantly outperforming Messi in the CL though! (much will depend on the Milan game of course)
Thanks for those stats, that's some good research.
It's obvious Messi is "more smartly" scoring goals than he was doing it last season. And as you said, goals scored in away games probaby have biggest impact on the record. If I remember well, it took him lots of games last season to score his first away goal in la Liga.
But still, him scoring "only" 5 goals in this years CL campaign is also saying he isn't playing his best football, since I don't think he will outscore his last year's CL record. He needs to score 9 more, because he scored 14 last year, and he has 6 games left. Of course, that is if they go past Milan, and go all way through the final. And even in that case, he will have 2 more apps more than in last year's campaign. Anything is possible, especially since we are talking about best player in the world, but I don't see him reaching that figures, especially now when Barca are playing pretty much without proper manager.
You are probably wondering why I am using his CL record into debate - I am just comparing his record through different competitions, because I'm sure you would agree that CL teams are better than the ones in la liga, and him scoring "just" 5 this year, is saying something. And even from those 5, he scored 4 against Spartak Moscow, and fifth goal came against Celtic. He hasn't scored in game against Milan, Celtic and both times against Benfica(admittedly he came on as substitute in second game). So, IMO if he had better opponents in La Liga, I don't think he would have achieved that record.
And btw, before someone came in with thinking that I am turning this into "Messi has become shit debate", I'm big fan of Messi