That's pretty subjective. You'll have to find your own answer. The basics are pretty simple, though. Aside from nicotine, e-juice is generally composed of PG, VG and flavorings. PG carries the flavor and provides throat hit. VG makes clouds (and is slightly sweet on its own.) The flavor molecules add the taste. Pure PG juice gives good flavor, but the cloud is thin and grey, and the throat hit is fearsome. Pure VG makes dense white vapor, but it doesn't carry flavor well, or wick well, because it's so viscous. The reason some DIY juices have to "steep" after mixing is because they are high VG. Thus, on the juice side, a combination of PG, VG and flavoring must be found to provide the users required amount of throat hit, flavor, ability to wick, and density of vapor. This ratio is interdependent with equipment considerations, about which more later.
Watts, essentially, are a measure of what engineers call power and physicists call work. They are not strictly electrical in nature-- James Watt did steam engines, not generators-- but can be used to measure power in any form. Watts are identical to horespower, except smaller: 1 HP = 745.7 W. Therefore, whether vaping or cars or nuclear reactors, more watts = more power = more work done. All other things being equal, in vape gear more watts means more vapor produced per time period. Whether this is thin, grey, throat-hurting PG vapor or dense, white, make-me-cough VG vapor depends on juice. *How much* more vapor depends on the efficiency of the gear.
So to gear. All attys have a vape chamber or barrel, the place where the coils and wicks live and where vapor is produced. A tank has this chamber topped by a chimney and surrounded by a tank containing e-juice. A dripper is essentially nothing more than the chamber, with no chimney or tank. Drippers are inherently more efficient than tanks. Partly so because of the lack of a chimney (where vapor is flow restricted, and cools and condenses) and partly because they are bigger. Aside from this more vapor is made by increasing the coil surface area-- vapor is only made at the coil surface, and then only where the surface is wet-- and increasing air flow to carry away already produced vapor, thus clearing the surface of the coil for more work. Drippers are more efficient because they hold more and bigger coils, and can supply greater air flow. Wick is also important, but does not make vapor itself. Rather it transports juice to the coil so it can, and it has to be able to provide enough for the coil to max out on vape.
Finally-- why this takes trial and error-- is all these factors are interdependent. If you change one, you have to tweak them all to maximize system vapor throughput. Anyway, there's a rough overview, I hope it helps you understand. There is no "right answer" or cookbook solution.
