...and I'm saying that some people can't really make the distinction because the movies feel different enough that it's not as easy to directly compare them. They're both good movies, so you have to compare them on their merits. I could say that Alien/Terminator does a good job at establishing a feeling of horror/suspense and Aliens/Terminator does a good job at establishing good action set pieces and memorable moments.
Although, in a sense, you could argue that both aforementioned sequels showed that you can change the formula a bit and still end up with a good movie. That's like an extra bonus for them.

All in all, we should be kind of happy that it's hard to choose.