Two-Piece or Three-Piece E-Cig?

At its simplest, an e-cigarette has two pieces, a battery and a cartomiser (like the ROK, JAC, or Liberro).  Simple.  You screw the cartomiser into the battery, and you're good to go.

The cartomiser can be refillable (the JAC or Liberro) with e-liquid, which will save you even more cash.  ROK say their cartomisers are not re-fillable - they are, but the results aren't that great, to be honest.   Their cartomisers (in virgin form) are great though.

A three piece e-cig is different.  The constant is the battery, but, instead of a cartomiser, you get an atomiser (which does the work of turning the nicotine into vapour).  In addition, you use, with the atomiser, a  cartridge,  pre-filled or empty (both of which are re-fillable), or a 'tank' (see the 'Jargon-Buster' for an explanation of these terms).

In my view, the best option for a beginner, is a two-piece.  It's quick and convenient.  After a while, you might want to try re-fillable cartridges or tanks.  That will save you some money, as you will be using e-liquid' although, as I have said, cartomisers are re-fillable too.  For myself, I have tended to avoid tanks - whilst they are very good and will suit some, I like the convenience of cartomisers - you still get to save lots of money and can of course refill them with e-liquid.   Also, tanks have been fiddly to fill with liquid - BUT - the new vision clearomisers used by JuicyCigs, eXhale and JAC (available as an option with the VGO) make refilling a synch!

uppus datus majoris:  Now that we have the new so-simple-to-fill 'vision clearomisers' (a new, better kind of tank) I am getting fonder and fonder of tanks.

For a more experienced user, a 'tank' may be the way to go, and all of the bigger cigs here are good for that. They all give good 'vape'.   Even so, regardless of levels of sophistication,  simple 'beginner' e-cigs like the ROK mini and the  Liberro Realis  are very hard to beat in terms of the vaping experience, and the JacVapour is an excellent 'compromise' between a big-bat model (some of which can be ugly) and a lookie likey (with a battery that might need a fair amount of recharging), without compromising anything.

Concluding, a two-piece is simple and gives a great vaping experience, but works out more expensive if you stay on pre-filled carts, and you may want to move on to something with a more powerful battery, after a while.  A three-piece with empty (or re-fillable) carts, or a tank, is cheaper in the long run (because you will be using liquid to refill), but you often have another piece, so, on the face of it at least, another bit to go wrong or that will need replacing.  I advocate a two-piece for beginners, and a three-piece or clearomiser-bat combo for more experienced vapers.