Atomizer: The heating unit only. Has no provision for supplying juice to the coil.
Cartomizer: An atomizer coil surrounded by a filler material to hold jice and deliver it to the coil.
Clearomizer: An atomizer coil isolated from the juice which is held loosely and wicked to the coil. Usuall in a clear housing.
Tank: A carto tank holds a cartomizer or clearomizer and feeds juice into the carto or clearo through a hole in the carto/clearo housing. An atomizer tank contains a heating coil as an integral part of the tank assembly. It does not use a separate carto or clearo like a carto tank does.
Variable voltage allows you to vary the heat produced by an atomizer coil. Some juices taste better when vaporized at different temperatures. With a fixed voltage device, the only way to change the heat is to change the resistance of the coil. With a VV, you can use one resistance of coil and change the voltage.
An eGo-T uses a one piece atomizer and cone assembly. An eGo-C uses an atomizer that screws into the bottom of the cone. In both of them, a cartridge is used to hold the juice and deliver it to the coil. The cartridge fits into the end of the cone. IMO, they are both a feeble attempt to revive the old, obsolete, atomizer/cartridge system of the old days. Delivering juice to an atomizer by means of a cartridge is always a substandard solution and fraught with problems like leaking, insufficient juice delivery and burning plastic tastes. The atomizer/cartridge paradigm became obsolete the day the cartomizers were invented.
If you want a change from a Smoov, you need to decide what form factor you prefer. If you must have something of a similar size, there is really no upgrade path. If you're o.k. with a larger unit, you need to decide how large and whether you want fixed or variable voltage. Then, you must decide whether you'd prefer a sealed proprietary battery where you throw away the whole ecig when the battery wears out, or generic batteries that you change like flashlight batteries.
If you want sealed batteries and can handle something a bit larger than a cigarette, a kGo is a good fixed voltage unit. Paired with a Vision Ego Cartomizer (Stardust), it performs far better than either an eGo-C or eGo-T. It's the logical next step up from a cigarette lookalike. If you want a disposable variable voltage, the Twist is available. But Twists are enormous for their capacity and you'll need 2 of them. A 650mah Twist is as large as an 1100mah kGo. A 1000mah Twist is as long as a 3000mah Lavatube. A pair of Twists also cost as much as a Lavatube.
If you want something with separate batteries, that is not disposable, there are a million "mods" to choose from, in both fixed and variable voltage. Your budget will probably be the deciding factor.