Maybe this should be a comment, I'm not sure.
Nothing in the Hobbit movie trilogy requires knowledge of the Lord of the Rings trilogy. It obviously helps if you already know a bit about Middle-Earth, but it's not necessary.
Having said this, there are characters in The Hobbit that come back in The Lord of the Rings, characters that you might think will die at certain points in the movies. This uncertainty is obviously completely irrelevant if you watch The Lord of the Rings first.
Another thing to keep in mind is that The Hobbit is a "fun" story. It's a jolly adventure, almost silly at times. The Lord of the Rings is a much more dramatic story, it's not funny. Starting with The Hobbit, it gets more and more sad, while starting with The Lord of the Rings leaves you with a happy felling at the end of it all.
So in conclusion, it's really up to preference. I would watch The Hobbit first, because it's an easier "getting into it" trilogy, while The Lord of the Rings is a lot heavier.