Regulated Mod, presumably with boost(not the setting), [they all have buck].
120W / 2 batts = 60 ea
"Max Amps Per Battery = (Max Wattage Per Battery / Minimum Voltage Per Battery) / 0.95"
(60w / 3.2) x .95(% mod efficiency[could be less]) = 18A pr batt.
P26 & 28A both ave plenty of headroom. 20S even more but at the expense of shorter run time between charges.
There's no wrong pick.