Default in Pleco is Song, yes. Kai is experimental / not heavily checked - based on the open-source
Make Me A Hanzi project that also powers Inkstone - plus, while we feel pretty confident that its use of Arphic's open-source Kai fonts is legal, there are indications that Arphic themselves might not think so, and so we're reluctant to default to using it while there remains the possibility we might receive a C&D letter from them + be forced to abruptly pull it from our catalog.
But we're working on a brand new set of Kai stroke order diagrams, which will be more thoroughly checked, cover many more characters, and not have any licensing uncertainty around them, and once those are ready they'll probably replace Song as the default (also saving us some royalties in the process). Probably not until after 4.0, though. (the licensing is in place, but we haven't found the spare programming time to build the editor app which the people we'd hire to key in / check them would need to use to create them from the font we licensed)