Market share does not really mean anything when your comparing an Apple (Unity) to Beef (UDK), The reason is if you have a read of the features you should see one is clearly superior and the other just limits you.
Unity ( single thread ) <<< SERIOUSLY i have seen modems with quad core processors.
Look at the games made with unity which many of them are all limited by the engine.
UDK ( thread safe ) << how many cores you got.
look at the games that are made with UDK all GRADE A
Choice is yours use a limited engine or use an Unreal Engine…