Empires has a new customization option if you fancy that, however the story concept is essentially nonexistent even for a Dynasty Warriors game. Your only option is playing the Empire Mode, and while you can play as a ruler or officer, there is virtually no difference. I have yet to play the Stikeforce, so cannot comment although I suspect it is generally identical to every other Warriors game Koei makes. Their inability at development is almost remarkable. Nonetheless pick either for a good "cut lose" game, however if you desire something with a stronger sense of substance or lasting appeal, look elsewhere.
DW is a great time killer or a stress reliever, it is not much else.