I have an 87 camaro with a 305, found two potential donor motors. One is a chevy 350 and one is a pontiac 350.

Are there any real differences?

 

YES... use the chevy. There are millions of options and parts out there for it.

 

Back in the day when each division actually made their own engines your answer may have had merit , but , our third gens have the same identical engine year to year whether Pontiac firebird or Chevy camaro . In other words , the very same parts that would fit the 87 IROC's 350 would bolt right on to the 89 GTA's 350 , unlike the old days of the engines not having any interchangeable parts . Most refer to the elimination of the different engines and adoption of the one engine fits all applications as the "corporate" engine strategy GM went to with our cars .

Hopefully it's the 89 cause it's two years newer , but by all means grab which engine is in the better mechanical condition regardless of being from a GTA or an IROC .....