The main difference is the technique used to vape. A mouth to lung tank is best for ex-smokers wanting something that can be dragged on in the same way you would a conventional cigarette. You do this by drawing vapour into your mouth first then down to your lungs, they typically deliver more flavour than dense vapour. Sub Ohm tanks on the other hand are beasts at producing huge amounts of vapour, and come with super low resistance coils (less than one Ohm).