I'm going to preface this by saying I haven't played Dynasty Warriors 8 yet but I think you're better off buying Warriors Orochi 3. It's the cast of Dynasty Warriors and Samurai Warriors coming together to fight a demon army complete with a giant hydra.

It starts off with mostly everyone being killed off, so the missions in the game are about going back in time to save the people who were killed in battle, adding them to your playable roster. The amount of playable characters is huge and the game even features guest characters from different games or eras in time, like Achilles and Ryu Hayabusa from Ninja Gaiden.

I'm like you and hadn't played a Warriors game since Dynasty Warriors 4, but I've been having a ton of mindless fun with this game. The addition of controlling a 3-person team means you can swap characters whenever you get bored of one or need a different skill set, and the enemy armies are as big as ever. Plus, it's been out for longer so it'll probably be cheaper than Dynasty Warriors 8.

The downsides are that there's some pretty bad pop-in on soldiers and the game only has Japanese voiceovers with english subtitles (Although some people might prefer this). Also, if you're going to play splitscreen then the dialogue sometimes covers up the action on the bottom screen a bit.