Not going to lie, I have no idea who this Horner guy is.
I'm an enormous fan of Glass, though. The reason why is because of how he controls density and texture in his music. Also, at first hearing, it sounds so repetitive, but when you've been listening for a while, you start to notice the subtle changes.
In fact, with his piano pieces and some orchestral pieces, I can identify what piece it is after listening to maybe 30 seconds to a minute. His pieces are not all identical, and he's anything but lazy; he's just precise.
Einstein on the Beach is actually one of my favorite operas. I've been in love with it for nearly 8 years now. Akhnaten, surprisingly, took me longer to love. In fact, at first I hated it. It was the first thing I'd ever heard by him, and I thought it was horrendously boring after the funeral scene.
Later, however, I came to love the rest of it as well. Especially after I started noticing the small aural changes that are, in his music, so big.
I really do believe he's an acquired taste.