It rather depends on what you use the bike for most of the time...
If your ride a lot in traffic, in town, or live in a hilly area 19T...If you live in a less hilly area and do more main road riding in more open country I'd go for the taller gearing of 20T...Ian
I have found out that it was very specifically mentioned in the contract for the Dutch Army WM21's which sprockets should be fitted to the engine and gearbox (both 21 teeth). I will add a photo I have made from that contract and from the BSA partslist for comparison. The contract is in Dutch, but the gearbox ratio's are easily compared with the 1940 partslist.