Liga MX is better than MLS. I'm sorry, but that's the truth.
Take the CCL for example, MLS has won the tournament twice (1998 and 200) Liga MX teams have won the tournament over twenty times. Since the tournament has switched to its current format, Liga MX teams are the only ones to win it. Last year's final was a direct clash between Liga MX and MLS. While it was closer than most expected, the Liga MX representative (club America) beat the MLS representative (Impact Montreal).
Let's look at tv availability in the United States. (Excluding live streaming services)
Liga MX:
Univision
Univision Deportes
Televisia
Telemundo
ESPN Deportes
TDN
NBC Universario
Unimás
Azteca America
Fox Deportes
MLS:
ESPN2
ESPN Deportes
FS1
Fox Deportes
Few local channels for the local team
When a foreign league (Liga MX) gets more coverage than the domestic league (MLS) I think it's safe to say that the foreign league is superior to the domestic league.
Bonus fact:
Liga MX also sends teams to compete in Copa Libertadores. This past tournament Tigres went all the way to the final before losing to RiverPlate. Liga MX sends their top teams from it's two seasons to the CCL. This means that they send the best of the rest to compete in South America's biggest club tourney. And they almost won.