Isn't UE4 free for all use cases under a million dollar of revenue ?
I couldn't imagine solo dev on an Unreal Project .
The available learning resources for Unity are much better than they are for UE4. I want to add X to my game, I Google it and find ether an asset or open source project within minutes .
For my current project I found a half dozen templates to start from. Everything in UDK was ether half done or abandoned.
Of course both , or even rolling your own , are fine.