The primary differences between BAe Harrier IIs and McDonnell Douglas AV-8Bs were their avionics and weaponry. Both the UK and US had slightly different preferences in what they wanted.
Both variants were progressively upgraded.
BAe: GR.5 > GR.7 > GR.9
MD: AV-8B > AV-8B Night Attack > AV-8B +
Regardless, this has no relation to the topic at hand. So please take it to the aircraft section.