I've been on this fence for a while.
From everything I've read, and all of the opinions I've heard, and all of the people I've talked to, this is what I've managed to work out.
Nautilus vs Aerotank vs 30S
The competition between the Nautilus and Aerotank seems to be pretty evenly split. About half prefer the Nautilus, and half prefer the Aerotank. That makes choosing between them difficult unless you can try both out yourself. Otherwise, you have to pick one based on other factors.
The primary other factors are cost and reliability.
Obviously the Nautilus costs more. The heads are twice the price as well. This wouldn't bother me if the heads lasted twice as long as the Aerotank, but from what I can tell they often last less time. It's not unusual to see people with the Nautilus only getting a week or so out of a head.
The Aerotank is cheaper to buy and cheaper to buy heads for. The heads seem to last longer, often 2 weeks to a month, IF THEY WORK. The biggest problem I've seen with the Aerotank are the number of heads that are DOA or fail within a few puffs. Kanger's problems with their heads don't seem to be limited to the Aerotank either, I just bought a 5 pack of T3 coils, and 2 of those are DOA as well.
The iClear 30S seems to be the dark horse here. Nearly everyone I've talked to that has one to compare with the both the Nautilus and Aerotank says the 30S has better flavor. The heads are comparably priced to the Aerotank heads, and I have yet to get a single head that didn't work.
However, the 30S lacks the adjustable airflow which is what a lot of people want these tanks for. Fortunately my Nemi has adjustable airflow so the tank doesn't have to. (I prefer the draw to be a bit on the tight side anyways) Also the tank is plastic, and while Innokin says it's designed to withstand tank cracking juices, I haven't been brave enough to put that to a real test yet. Use tank cracker juices at your own risk.
I'm still trying to decide on an Aerotank or Nautilus, to see if either offers me a better experience than the 30S, but I'm REALLY stuck on trying to decide which. The more expensive but slightly more reliable Nautilus, or the less expensive Aerotank with heads that may or may not work when you buy them. Decisions decisions....