Well, strictly speaking, the standard of Premier League teams is much better than that of MLS teams - and I say that as a diehard MLS fan (Go NY Red Bulls!) In past years, MLS teams have gotten the odd win over an EPL or other European team, but this result is par for the course in these pre-season friendlies, where the MLS teams are usually pretty over matched.
As for the question of how good Man United is, we'll see when the Premier League season starts. Likewise, the true quality of the Galaxy we'll find out come playoff time.
JustaPerson, aren't you glad you asked :-)