I don't know compared to Unreal but Unity has several hidden performance issues. I am currently working on Bullet Boy [1] and we had several obscure problems that you have to fix with a lot of trial and error. Specific combinations of components and attributes can cause frame rate to drop because of the internal organization of the engine (moving colliders without rigidbodies comes to mind).
That being said, it is really easy to use. The asset pipeline is great and artists learn to use it very fast. It works really well for small teams.
My recommendation is that you should consider it if you are not planning to make a very performance intensive game.
Regarding Unity vs. Unreal, it looks to me like Unity is better for targeting mobile devices, at least if you're planning to deploy this year. Last time I played Tappy Chicken, it devastates my batter and makes my phone hot enough to cook on. Unreal has amazing advantages for the desktop, but it doesn't look to me to be a good way to make mobile games.
If I'm wrong about this, someone please tell me, because I'd rather use Unreal, but I need to deploy mobile, so I'm using Unity.
That being said, it is really easy to use. The asset pipeline is great and artists learn to use it very fast. It works really well for small teams.
My recommendation is that you should consider it if you are not planning to make a very performance intensive game.
[1] http://bulletboygame.com