I've had the opportunity to pit models that were powered by both on certain occasions and honestly I couldn't tell much difference. However, I wasn't a regular F2C pitman at the time so didn't have extensive experience in that regard. Within the confines of a fully cowled F2C model the rear exh. engine was quieter though. I think you could regulate the heat better with the side exhaust engine.
The fellows I knew that used these engines upgraded in fairly short order with every new development that Henry came up with. All of the rear exhaust engines I've personally seen were either Steel or ABC P/L variants. By the time the Nelsons were using AAC's most everyone I knew was using the Side Exhaust engine.
I use the the Side exhaust versions myself, both Front and Rear intake for F2CN and testing with other models.
Les