Do you have the kit with Nautilus 2 tank, or just the Zelos mod powering a different tank? Also, are you looking to stay with an internal lipo battery, or would you consider a mod with replaceable batteries? Advatage being, the mod lives on if the batteries need to be replaced. Are you using temp control? Answers really depend on how the device will be used. However, $100-150 gives many good options.
Yes, a dual battery mod and I'm guessing a tfv8 baby beast, big baby beast, or cloud beast is much more potent than a Nautilus 2. Some like the mouth to lung nature of the Nautilus because it closely mimics smoking. The Smok tanks are mostly direct lung inhale, much more vapor production, so use less nic than with a Nautilus to get same effect.