Cartomizers have a cartridge for storing
juice, and an atomizer for heating the
juice and producing vapor, all built into one piece. These are usually meant to be refilled a few times and thrown away.
Atomizers are simply the parts of a system that produce the vapor. Usually a cylinder with a heating coil, and some sort of bridge that can optionally make contact with an independent cartridge that holds juice. So, atomizers are either combined with an extra cartridge part, some sort of juice tank, or dripped directly upon.
To me: TH is slightly improved through cartomizers; however, for the separate atomizer-carto or direct dripping setup, it can depend on how you pack carts, the juice you use, and the type of atomizer.
It's hard to say that one system is better or worse than the other. Cartomizers are less of a mess to me, but it's probably best to try some of both and form your own opinion.