Two completely different animals.
The smart can do a lot of things the edge can't, like change tire size and check/clear check engine lights/codes.
The Edge is going to make more over all horsepower (almost double with the wire tap).
Personally, I would start with the Smarty. You can always stack the edge on later (or other options), and then you have the ability to check codes and clear them. Use the search function and you should be able to spend a few days reading all the information.