Mohammad Daoud looks at Dynamics GP Vs. Dynamics AX. While I generally agree with his comments, I think that GP is more customizable than people with less effort than people give it credit for. It also has much strong financials.
Mohammad R. Daoud: Dynamics GP Vs. Dynamics AX
Archived post from 2011. Dynamics GP and the links it points to have changed since; check current documentation before relying on version-specific steps.