Also, and unlike the 737, several major components have different suppliers on the 320. From a pilot's perspective the two biggest ones would be the motors and the FMGCs (Honeywell or Thales). This makes mixing Airbus fleets a little trickier but not impossible.

Useless trivia: JetBlue's 320s are optioned virtually identically to UA's. Back in 1999 the leasing companies did not think JB would succeed so they wanted airframes with lots of existing North American parts support for resale value.