it depends on what you want to see. If you want straight up bike porn, then get NWD 8. It's sick. But if you want a more in-depth perspective into mountain biking with a lot of great, progressive riding, get Seasons. I've seen both, and both are amazing. I'm personally a big fan of The Collective, so I'd say Seasons.
Seasons was amazing, so was NWD8. But I'm going with Seasons because it feels so "real" plus it was more DH oriented rather than slopestyle and the cinematography was amazing! NWD8 shows a bunch of pros pushing themselves to their limit, whereas Seasons had much more of a lifestyle feel.