For the primary series, Moderna dosing is 100mcg per 0.5mL versus Pfizer's 30mcg per 0.3mL dose, so it isn't necessarily surprising that Moderna's has been slightly longer lasting. It's also the same reason people tend to report Moderna takes them out for half a day or so post administration more often than Pfizer does.
Interestingly, Moderna's booster dose is reduced to 50mcg per 0.25mL, but Pfizer is sticking with another 30mcg per 0.3mL shot.
I wonder if Moderna's will become the vaccine of choice given
HC just authorized a two-month shelf life extension for it.