I'd expect Dart Amat to be available during January, same as DMM version, since limited himes are generally associated with some specific time of year.
Hades definitely worth it, most powerful dark hime other than those god-tier imo; although my favorite, and also DMM players' favorite, is Osiris.
Hades can be gotten with a miracle ticket, where as Dark Amat can't. Utility-wise, they both take a role that can't be replaced. I guess it comes down to whether you're willing to wait for half a year...