I want both, When using a 21700 mod, I use a 21700 batt since there's no reason to use an 18650 IMHO as I rarely use 21700 mods out and about. But for out and about, I prefer an 18650 mod for less weight and smaller size of mod.
EDIT TO ADD: That said, dual 21700 mods have a somewhat good demand - it's a niche but a good size niche. Squonker mods are also needed. Chinese market has seemed to just about abandoned both.
I almost exclusively used dual 21700 mods. And it's not that I vape at high wattage I don't on average about 25 watt because I don't like changing my batteries and I don't like cycling my batteries a ton of times.