Volibear or Sejuani?

Both are fun and strong, but Volibear is better right now so I'd go with him. Plus you can use Volibear top if you want to switch things up.

I love playing Sejuani. You provide an immense amount of lock-down and good consistent damage during teamfights. In addition, your mobility and CC provide strong ganks.

However, I am biased; the Poro Rider Sejuani skin is amazing.

Voli's getting nerfed in the next patch (sadly for me, as I've been maining him this year). I wouldn't buy him until it shakes out how good he'll be after that.

I have mastery 5 on both champs though, and certainly find them both fun. Voli is slightly easier to carry with, but harder to be effective when you get behind. He needs to be in combat for several seconds to do his thing. His nerfs are aimed at making getting ahead harder, and I'm not really sure how hard they'll hit. The trick with playing him is just understanding when you have the MS + tankiness to be worth going in on a lane-gank / teamfight.

So much of Sejuani's power is in her ult. Winning games with her is a lot of just figuring out how many of her ults you can hit. 85%+ (and lanes that can hold their own until you're level 6 and have useful ganks)? That's a win. 50% hit rate with Ult (or lanes that get behind before you get it)? Probably a loss.

I wouldn't try Voli (and especially not after the 6.13 nerfs) without AS reds. Ditto Sej without mpen reds. The rest of the runes don't really matter much between intelligent choices. I run MS quints on both (though champion.gg says I'm fairly alone on that). You could get away with general tank runes for everything else & be fine.

I probably find Voli slightly more fun (but that could be the winrates talking). I'm going to be a bear and eat everyone. It's a blast. No skill-shots to worry about if your connection is bad. Sej plays a lot more like Amumu--some big highs, but some missed skill-shots followed by walking around without much useful to do.

Volibear is probably the best jungler in the game right now. I have mastery 5 on sejuani but I rarely play her as she will get counterjungled too hard.