Generally they were both bred to fulfill the same niche only in different countries. It would more depend on what exactly you expect from them. Then,it would matter greatly which line of birds you chose. Some breeders have pretty much chosen conformation and other external characteristics to rule their roosts. These would be more show than go so to speak.
Many commercial flocks have been bred soley to be productive,more go than show.
What ever you decide it will be a compromise. If you butcher them the only real difference is that Rocks have yellow skin and Orphingtons have white skin. You won't notice this for long though as all fried chicken has brown skin. Orphingtons come in four colors Buff,White,Black and Blue. Rocks come in seven standard colors and some others as well.