The majority of 2-cell capable lights use buck-only drivers, and drop out of regulation when Vin < Vf. You can probably count on one hand the number of buck+boost lights that are equally bright and equally well regulated running 1 cell versus 2 cells.
So what that means is, 2x16340 will run brighter, and flatter regulated than a 1x17670 or 1x18650.
There is a
remote possibility that module is buck+boost regulated, but I strongly doubt it. It would be a VERY unique and ground-breaking product if it were.
anyhow... cant go wrong with the standard recommendation...
http://www.lighthound.com/AW-RCR123a-Protected-750-mAh-Battery_p_114.html
http://www.lighthound.com/Ultrafire...3-37-volt-Lithium-Battery-Charger_p_2279.html
You'll need spacer adapters though
http://www.lighthound.com/15mm-Charging-Spacer-Aluminum_p_25.html
"Is it better to go with 2x16340 or 1x 17670?"
Depends on what you value. If you want brightness, flatter regulation, don't mind shorter run times and the complication of extra cells go with 2x16340. If you want longer run times and the simplicity of a single cell but don't mind direct drive and lower output go with the 1x17670.
FWIW I wan my DX:11836 with a 1x17670 and it was....:green:
. It was plenty bright, but the lack of regulation and subconsciously knowing I was starving the DC-DC of voltage interfered with my enjoyment using it. I modded the module with a 3.7V specific driver and its perfect for me now.