Here are the test results of the AW IMR 18650 1600mAh vs the 2000mAh. Test was performed at 2.5 amps which is the approximate draw of a LR 1.5ohm atomizer. The winner? It's a tossup.
Through most of its life, the 1600mah does hold slightly higher voltage under load .
Though total tested mAh of the 2000 was almost 400 more, only 200mAh of it was "useable" and only when using vari voltage/wattage devices (assuming we change out the battery by 3.3volts which is when the Kick by Evolv automatically shuts off). The other 200mah was gained when the battery was under 3.3 volts.
My opinion. If you are running vari wattage/volt mods, run the 2000mAh as you will gain maybe another 1/2 to 1 hour run time.
If running mechanical or non VV mods, run the 1600mAh for its added ability to hold voltage under load. If you change your batteries out at 3.5 volts, you gain no mah by running the 2000mah and it can't hold voltage as well as the 1600mAh.