Are you talking about the original show/toy line? Because as I recall they were both about equally loved by the kids at that time (I should know, I was one of them). Both shows had an element od silliness to them, though I'll admit MOTU at the time was maybe a little sillier. But as for popularity, I'd put them about equal.
Well, i wasn't born in those times, neither did i grow up with those cartoons, i actualy find He-Man realy terrible for it's time, expecialy when compared to stuff that was killing it outside the USA at the time, like Saint Seiya, Heidi, etc. But as Pink Ranger has said, Transformers has had a longer sales cycle than He-Man, He-Man's came and went like a lightbolt, i believe there's even a book about the rise and fall of the brand.
Regarding their silliness, the thing is that "He-Man" by itself isn't a nae that can be easily taken seriously, don't get me wrong, Transformers is silly by itself (cars transforming into robots is inherently silly), but there's a reason He-Man has become such a large target for gay jokes. Transformers also had better animation than He-Man and lasted for longer, then after Generation 1, you had the whole Beast Wars brand, even when the franchise wasn't as successful, it still had those animes that gained some popularity at their time. He-Man's revival in 2002 was well liked by old fans but was not a comercial success.
Transformers realy is easier to make a hit of, you have a neat concept of robots who disguise as cars, with He-Man, the concept is harder describe, it's pretty much a merging of different concepts, both science fiction and high fantasy.
Now when it comes to the movie, first off it had Michael Bay's finger prints all over it (slow-mo panning shots, explosions everywhere, etc). And I found Shia Lagoof's acting to be horrendous. I also didn't care much for the way Sam and his parents were written. Now the movie had some FANTASTIC VFX, but great effects don't necessarily make for a great movie. Just ask anyone who suffered through the Star Wars prequels.
Dude, i never said the Transformers films were good, i just argued that the franchise was easier to turn into a hit, expecialy with the first film being godfathered by Spielberg himself.