Kha'zix is good at snowballing, dropping explosive burst and resetting, split pushing with stealth/leap, AOE farming, poking...very very strong champ for solo queue.
Volibear is a great tank, has deceptively high damage, is fun to play, and can have some pretty monstrous ganks. unfortunately he's easily kited and is surprisingly vulnerable when his passive is on cd unless you're fed.
i would say khazix, its very difficult to stop a smart khazix. even a smart volibear can have all the momentum ripped away if he gets hit by a random slow.