Both are historical dramas, albeit from different eras. The reasons are different, but the female main characters both live disguised as men. The Tempest is a sort of fantasy story loosely based on real events while Onna Nobunaga is an alternate take on history and a re-writing of a real person. There's romance in both, but The Tempest is more character-driven. Onna Nobunaga is a bit more realistic in some aspects and there are lots of fighting scenes. Onna Nobunaga is shorter because it's a two episodes special.
