Another take on the OP.
Basically with li-ion batts you have two choices for single batts - 3.0V & 3.7V. These are
nominal voltages, by which I mean that fresh off the charger voltage will be a bit higher. So if you like vaping at these voltages, and with different ohm atties/cartos available, you can get to a general "sweet spot"; the higher mah that you can get, the better. This will mean longer times between needing to charge the batt.
If you use 2 batts in series for higher voltage vaping, you will still be better off with higher mah batts for the same reason.
And like Nicks pointed out, 2 batts in series doubles the total voltage while the total mah is the same as a single batt. 2 batts in parallel doubles the total mah while the total voltage is the same as a single batt.
The mah (and the specific li-ion chemistry) is also an indication of how much current can be drawn from the batt; but that's another topic
