I say all three. I have a regulated voltage mod, a mechanical mod and a Kick 2 variable wattage mechanical mod. The regulated mod allowed me to go explore re-buildable atomizers, rba's, since it had an ohm meter included, and allowed me to output up to 15 watts of power. It allowed me to learn how to become a competent builder of rba's. Then I started to explore mechanical mods, love the simplicity and elegance found with mechanicals. I also bought an Evolv kick 2 with my first Mech purchase, because I wasn't ready for the sub ohm zone quite yet and wanted to get at least 15 watts to vape at. Then as my experience and knowledge grew I began my sub ohm adventures, making sure I had the proper batteries, ohm meter and tools.
Not all atomizers are well suited for sub ohm vaping, many work best at the 15 watt range, for those I use my regulated device and a mech with the kick 2, but for my rba's that are designed for sub ohm, I use a straight mechanical and 30 amp IMR batteries.
Mech's, rba's and rda's are advanced vape gear, knowledge and care must be used to give one the most safe vape.