What's the difference between Dynasty Warriors and Warriors Orochi?

Both series look like the exact same thing. Are there any differences? I've never played either series before but I'm interested in trying one for some fun couch co-op.

What's the difference?

Does each series have a particularly good installment? Or perhaps an installment that was worse than the rest that I should avoid?

Do the games play anything like Mystic Heroes? I loved that game back in the day.

Is there any skill involved in these games or is it just mindless, easy button mashing?

Thanks so much!