Are these fuel injected engines or carberated?
The turbo engine will loose less hp then the naturally aspirated engine will. The turbos will compress air and force it into the engine. Higher elevations will have thinner air. There for you do not get as much air into the engine. With today's fuel injected engines, air and fuel are mixed much more effectively then the older carb engines where. Therefor you will only loose around 9%. But the turbo engine will still loose less due to the compressed air. The more air and fuel you can get into an engine the more hp you create.
In higher alttitudes the same jar of air will have less molicules then the same jar at sea level. A turbo will create about the same amount of air at any altitude just because the air is compressed.