Z5 and Z2 are both just sealants. Neither is a polish, and neither will remove swirl marks. Z5 will *hide* swirl marks, because it has fillers in it.
I use Z5 on dark colored cars because, in my opinion, it looks better. It has a deeper, more carnauba-esque look. I do not generally use it because it fills, that's just something it happens to do. I use it for the look and durability.