Hey Jeremy. Welcome! First, thanks for ALL of your work on the Rayon thread! It has, perhaps, had the most effect on my vaping experience.
I, like you, was using two VAMOs in series configuration before purchasing my first iStick 20W. Battery life with stacked 18350s is definitely better than a single 18650 and provides up to 30ish watts at the coil. Not until I made my VAMO box mod with a 1600mAh, 7.4V Li-Po did I find an all-day-device however. And the box-mod is on the large side.
The things that make the iStick so brilliant are tiny size, ergonomics and battery life. The 30W & 50W are far superior to the 20W in both electronics and 510 connection. Most here are experiencing at least full-day battery life from both the 20 & 30W (up to approximately 6ml of juice). Experiences on the 50W are just starting to roll in. Many are reporting 2+ days on a charge. The chipsets on both the 30W & 50W have a flat DC output at 4v and above. Below 4V, PWM is used for regulation. Below 3V, things get kinda wonky. I wouldn't bother.
As for the series/parallel discussion... At up to 50W, we simply don't need the amperage from dual batts, either in series or parallel. The varying technologies, between buck, boost or PWM (or a combination thereof), are simply different ways to skin a cat. Theoretically, we can discuss the differing efficiencies amongst the choices. However, until we get to the $60+ chipsets (DNAs, SX-350s), the quality of the individual components in a chipset (ICs, CAPs, etc) have far more impact on efficiencies. The VAMO & ZMAX chipisets are more efficient at 7.4V than 3.7. With regulated devices, it's simply watts-in-watts-out. Ignoring chipset efficiencies, battery life is completely dependent on the Watt-hour (Wh) capacity of the setup. Two 3.7V 2200mAh batteries, in either series or parallel, produce/contain the same Watt-hours: 16.28Wh.
Good luck!