The 21700 lights I own are just a little too large to trouser carry. So I still prefer 18650. But if wearing a coat or carrying a bag i may consider A 21700.
I'm now a convert. Last two lights I bought were 21700's. One battery has a USB plug and the other charges in the light (via usb). My dozen or so 18650's lights now rest with my 123's and AA's. All part of being a collector.