Modern derailleurs work uniformly well whether medium- or long-cage so there is not a lot of performance downside to going long-cage and it gives you the flexibility to possibly go to a larger chain wheel tooth difference (smaller granny gear) and/or larger spread on the cassette (larger big cog) without buying a new derailleur. It's not just chain take up of course; the front derailleur would need to handle the tooth difference and the rear derailleur the largest cog as well. Either alternative or both might be found useful on a cargo bike where a "stump-puller" low gear might save a long uphill walk.