In series, you have a "weakest link" situation where the
cell with the lowest -real- A-h capacity will limit the
usable capacity of the entire stack. This is how sealed
lead acid batteries always seem to fail - 5 good cells
plus one that's got low capacity and high series resistance
once depleted.
My preference would be a high efficiency step-up converter
per battery, parallel at the outputs with current control /
limiting to enforce some reasonable current sharing but
tolerant of cells dropping out at any point. Such a scheme
would also permit adding capacity "chunkwise" without a
whole lot of rip-up.
But the original question only wanted to explore battery
wiring choices. There, you have concerns about wiring
gauge and weight / volume - are you better off sending
the power as voltage or current? At low voltage like
this, insulation is trivial but copper can be hefty, and
long runs definitely prefer voltage.
But the realities of battery capacity (and especially
aging) unevenness may pull you away from the plain
series stack on the discharge side of things, while
the charge / charge retention side might not like the
parallelling. So what's your deal there? Max run time
fresh off the charger, or max run time after a week
of sitting in the airport parking lot?