Pragmatic answer: why not just try both and see? They're both free, after all. When I started Unity it took me maybe four hours to make a very basic shmup, which let me get a feel for the engine. Unreal took me a little longer, I found it had a steeper learning curve, but a couple of days isn't a waste if it lets you find the right tool set for your needs. It's still quicker than waiting for us to tell you what to do. ;-)
...and for completeness: I ended up choosing Unity for my current project, but I've prototyped a possible future project in Unreal, and I may switch, just for the giggles.