There are differences between the wicks/coils of top-fed tanks and bottom-fed tanks. The Nautilus is a BCC (bottom coil) tank and the iClear16 is top coil. Bottom coils tend to last longer (in my experience) and can be rebuilt (meaning, you can take out the original wick and coil out and build a new one yourself) or you can buy replacement coils. Top coils, specifically the iClear16, provide really good taste and gurgle less, but you also have to be careful to keep the wicks that drop down into the tank wet at all times or they will burn very fast. So, this means constantly turning the tank upside down to re-wet the wicks and filling the tanks pretty frequently too. The iClear16 also has replaceable coils and you can rebuild them also, and the coils for each are about the same price, depending on where you buy. I have not used the Nautilus, but have used the Pro Tank, Kanger T3 and T3S, XJet, and others, and really do find that overall, the bottom coils tend to last longer than the top coils, but that's just my experience. Glass tanks are heavier, in case that is an issue for you, but they tend to last longer, can handle any juice while the plastic tanks are lighter.
If you are running through your coils faster, are you changing the juice your use? Or are you cleaning them at night? These can have an impact on the life of your coils. For example, dark juices/liquids can burn coils much faster than lighter juices.
There are disposable options also, but you may not want that as you mentioned you were not wanting to buy the whole top, but wanting to replace the coil only. But, there are disposable and cheap options out there.
Good luck!