Nautilus and Sion are two very different champs. Sion has way more offensive capability in fights, while Nautilus plays more of a tank role.
Nautilus has been one of the strongest junglers since his inception. Since Spirit of the Elder lizard has been removed, his mana pool/regen needs to be taken into consideration as he's completely useless without mana as far as ganking/teamfights goes. In spite of this, Nautilus is a monsterously-strong initiator and can completely incapacitate tenacity-less squishies with the help of a buddy!
Nautilus is the king of crowd control. He's like the leona of the jungle. Anybody who says Nautilus isn't a strong jungler simply doesn't know how to play him well. Nautilus is one of the most underrated champions in the game.
The only scenario where it wouldn't be appropriate to pick Nautilus is having a team comp that's already flexin' a lot of tanky crowd control and less-than-appropriate damage output. In these cases, you'd want to pick an asassin-type with great wombo-combo potential.
Hopefully you find any of this useful. I strongly reccomend buying Nautilus if you like to go H.A.M in the jungle and want to spice up your jungle-champion roster a bit.
ps: trundle is also cool and underrated jungle, buy him too