Considering the Unity Engine
Here's a good read for anyone trying to adopt a game engine. Someone sort of brought it up at the just concluded G3 Mobile Games Developer Meetup in Lagos. I must warn you, only slightly seasoned game developers will understand most of what is said. New and eager game developers might struggle a bit.
Note, the write up does not juxtapose Unity with the other engines. It just gives some instances of what unity is good at and what it is improving at along side other popular free tier engines.
If you are strictly looking at 2D, I'd say you should give Defold (King's own Engine) a try. I prefer it to Corona SDK, simply because an Editor can make your life easy. Will write a blogpost about this in the coming months.