My rule is to always use the shortest cage with sufficient capacity for the gear combination. There's only a marginal advantage, namely that the cage doesn't hang as low and is less likely to get damaged. There's also a weight savings but not enough to think about either way especially for a cargo bike.

But, before shelling out dough, take a moment to consider if you have enough gear range. If the shorter cage is at full capacity with the planned setup, you might opt for the longer cage, to leave yourself the option of expanding the gear range later on.

BTW- even though I prefer shorter cage RDs for their higher ground clearance, there's no meaningful drawback to longer cage RDs except their uncool factor on road bikes.