Battery life has more to do with how you vape than how many MaH it's rated for.
Yes, technically more MaH equals longer battery life, but when you compare 18650s there is only so much MaH you're going to get. If you sub ohm a 20 amp battery at 15 amps you're MaH rating will suffer alot more than if you vaped the same battery at 10 amps. The more stress you put the battery under will greatly influence the battery life. Also cell quality, not all batteries are made equal and even the best batteries will start to hold less and less of a charge over time.
In conclusion a single 18650 config anything over 2100MaH is going to be so similar it's not even really worth debating. If you're putting stress on the battery it's more important to look at the amp rating over the MaH.
PS: Any claims that a single 18650 can output 3000MaH are GREATLY exaggerated! Don't believe the hype when it comes to overseas MaH ratings. A very light vape on a HG2 between 5-10amps and it's capacity drops to 2800mAh at 2.80V for 5A and 10A. There are other batteries that only claim 2100MaH and last just as long.