Killis. Unless u breed a really nice pair of bettas. Like get a $40 pair of over halfmoons or something crazy. I personally couldnt give away 50 some betta splendens. If u want to just try to breed something, go with Killis.
go with the killies, the are in general not seen as often, and unless you're breeding high quality bettas or wild types, most standard bettas are cheap bread and butter fishes in the hobby