To quote a review I read on Unity (don't remember the link), "Unity is like having the keys of a Boeing 747 handed to you".
There's a lot of power there, but it takes a lot of time to harness it. I only spent a couple of hours playing around with it but I felt it was overkill for the kind of games I wanted to write. I just want to try my hand at some simple 2D games and go from there. And, based on the games I've seen in the gallery, there is a lot of room for me to grow with AGK.
I don't have a lot of time each day to work on this and AppGameKit, so far, gives me the biggest return with my time.
I also didn't like the fact that Unity took about 9GB of disk space!
Phil