One Man Team Game? 3D Game Engine

Title says most of it, so I'm just stuck between the unreal engine and the source engine.

Both have their development kits, I know source better than unreal, but not sure how much I need to know about unreal to make the game I want.

I'm basically making a Grand Theft Auto, Sleeping Dogs, Saints Row, clone or you know. And open world game, maybe not about gangsters, but you know just open world refers to gta and sr.

So which one should I pick? I want to be able to toggle 1st and 3rd person. That's the main focus of the engine, I need to be able to create an open world game with a storyline which I know I can do with source engine and seeing sleeping dogs I think I could, but might require scripting.