I'll knock this down because....

Since I last posted my question I've been reading up. In case anyone else would like an answer, KMOTGG is open-pollinated and will come true from seed. So it's worth it to collect the seeds for new plants.

As far as I can tell, both the asters I asked about are hybrids, so they won't come true from seed. So while I might get some asters, I won't know how they'll look until I see them flower. If I want the same plants, it's better to divide the ones I'm already growing.

If only one color and one variety were grown and they weren't hybrids and no wild flowers that can cross with them were nearby, probably. I try to chose my robust and favorite color from each variety that won't cross in order to save seed. I do this with sweet william, zinnia, poppies, snapdragons, squash, ect. In some cases, though, I simply grow one color one one side of the yard and another favorite color WAYYYY on the other side of the yard with my house acting as a divider. So far, it works.