Honestly the game you're describing sounds a lot like Fortnite (Battle Royale + Save the World). You should check it out if haven't already. This game was made in Unreal Engine 4 and looks beautiful in it. Unreal Engine will be easier to use than unity 3d but like you said unity 3d has a larger community and more assets available to the public.
However, what you said about "tutorials" doesn't really matter because both will have a lot of tutorials/guides available. There's a lot of really good walkthroughs such as "making an FPS in Unreal Engine 4" that has 30+ episodes each.
I prefer Unreal Engine because its easier to use, and if you don't want to you don't even have to touch any code. Also, you said that with Unity you can't view the source code which is the opposite way around afaik. You can't view it in UE unless you establish the project as being made in Visual Studio. UE generates code for you