I use Pico Squeeze 2 squonk mods, they're what converted me to using 21700 batteries. They came with a sleeve that allows use of an 18650 but I hated using it so started buying 21700s, certainly glad I did.
I recently bought six more Squeeze 2's directly from Eleaf while on clearance for $19 ea. They charged me $22 shipping so each mod actually cost around $23 but even that was a great buy. Would've bought more but that's all they had left, they're now discontinued.
Battery life and weight have always been important to me.
The old 18350's had to be seemingly constantly changed and were a PITA in the charger. 18650's were much better (up to 3000mah) but were marginal on lasting all day. (I don't own very many) While I've owned some dual 18650 mods over the years they were usually not the first mod I'd reach for because of size and weight. As a compromise I started getting 26650 mods. They would last much longer than the 18650 ones but then finding new/good batteries of that size became problematic I started looking at the new (at the time) 21700's.
I also own a dozen or more Pico Squeeze2's that take 21700's and a 1/2 dozen regular Pico's (also 21700) and a 1/2 dozen or more other brands that take 21700's. (I also got a VERY good price on my Picos!)
The 21700's last all day, are not to large or heavy and can be got with 5000mah capacities.
I get
Molicel/NPE INR-21700-P42A 45A 4200mAh Flat Top 21700 Battery's for $5.86ea.
g.