Here are my opinions:
1. The Unity 2D toolset is fairly new. I gave it a quick try and I found it a little bit confusing. I wasn't exactly sure how to create something to fit a specific resolution. It seems more geared towards creating a 2D view in a 3d environment, but I am not sure how necessary this is in your case. I am not sure if I would go with Unity 2D just yet because it is very much not proven.
2. Lots of games have a high quanity of art assets and animation. If you think that your game requires significantly more, then I would take a close look at your game design because it is likely that you are not designing it correctly (trying to create a game that is simply not practical to create).
3. None of the solutions will have much in the way of AI. You may find that one of them offers some form of pathfinding but I bet that is probably it. You will have to code this...
4. You will have to code your character and skill leveling systems and game customization no matter what you select.
You are going to be doing a significant amount of programming in any one that you pick. I personally would probably go with Game Maker but that is just because I know it the best.