Everyone seems to get fixated on the resistance number. It's not resistance that we are driving for, it's heating element size. And a larger heating element produces more vapor, can deliver more watts, and has more mass. Resistance it directly proportional to length, and inversely proportional to cross section.
And with that in mind, it really depends on what you're building for. On a mech, a thick, low resistance coil is going to have to be pretty small since the wattage is set by your voltage. Resistance is as much a factor as size and mass. As you go down in gauges, your resistance has to be proportionately lower in order to get enough power to it for it to fire properly, so there's a limit to how much you can benefit from having a thick, heavy, low-resistance coil. You have to take roughly as much length out as you add bulk.
That's where I think the whole resistance mindset comes from. It's a remnant from a time not so long ago, when the only way to get the power we take for granted now was through a low-resistance coil on an unregulated device.
That's also where the current-to-heat association comes from. There was a time when this was always true. I remember when people still thought that voltage could never give them the heat like current could. They hadn't considered that the wattage-mass ratio was the key factor because current was all that they had to work with.
That is to say that it really does come down to the wattage for your coil mass either way, though. The approach just differs slightly, depending on how you get your power. Going by resistance isn't necessarily wrong, it's merely an oversimplification of things.
Ramp-up time and wicking efficiency are factors, as well. Sometimes, a smaller coil that requires less power can outperform a larger coil simply because the wicking has an easier time keeping up across the smaller surface area, which also transfers heat more quickly without getting as hot overall, effectively making it able to displace more juice in a shorter period of time without overheating it, whereas a larger coil displaces a larger amount of juice at a slower rate. It comes back to that heat-transfer balance you were referring to. One must also consider that a coil can only move as much juice as the wick can, which is something that I'm sure tank users in particular can attest to.
I don't know why people focus on any one factor. There is so much to consider. But I suppose the easiest way to learn how all of the variables relate is to examine them one by one.
