Which Dynamics Platform?

I know this is a loaded question, but which Dynamics platform is best suited for which business?
I have been trying to figure out the differences between the products.
I had hoped that this would be documented somewhere, but I have been unable to find anything concrete (even from Microsoft's Dynamics site).
So can anyone help out with the difference between AX, NAV, GP and SL?
Or perhaps a site with information regarding the differences?

Thanks!
G