Going Oerlikon way avoids possible problems with too short the casing. Coupled with earlier availbility vs. French Hispano, let alone vs. British Hispano, this might lead to having 20mm cannons already in US arsenal much before the war. USA did bought license for the big Oerlikon, the FFS, albeit a bit late, and used it mostly on ships.
Oerlikon FFL or FFF might also be good choices for the RAF, meaning cannon-armed Hurricanes and Spitfires already in 1939. Going for FFL or FFF also allows for more ammo to be carried for same weight and volume alloted vs. Hispano, plus the same cannons were smaller and lighter than Hispano.
http://www.quarryhs.co.uk/OeFFL.htm
For the RAF, the
Vickers 0.50in is also an option, be it in fixed or movable installation.