RDA's can serve many functions, the biggest factors will the two biggest factors of what you are wanting out of the RDA
1) Flavor with Decent Vapor Production (Camp 1) - Flavor is produced best out of RDA's with a narrow or reduced chamber and a some restrictive air flow. Examples - IGO W V1, Helios, Derringer, Bambino, Veritas, Quasar, Atomic
2) Massive Cloud Production but Decent Flavor Production (Camp 2) - These setups have larger decks and chambers allowing for larger diameter coils which allow for larger wicking, that allows major liquid wicking and flow to the coil, more liquid vaporized quickly is more vapor, and have very, very, very big airflow options. Examples - The Mutation X series, Velocity, Dark Horse
3) Good Flavor and Good Vapor Production (a balance of both) (Camp 3) - These setups are in between Camp 1 and Camp 2, medium sized decks and chambers with medium but airy airflow that promote good flavor while at the same time giving good vapor production. Examples - Vector, Plume Veil, Tugboat RDA, Zenith