Read the Software Advice piece here - Microsoft Dynamics for Manufacturing – Understanding the Difference Between GP, NAV, SL and AX
Read Mark's Post Here - Differences between the Dynamics Products
A little while back(from 2006) I had read a nice comparison about the products by David Singleton at the DynamicUser forum -
"The strength of Great Plains is that is runs out of the box. It has never been strong on its modifiability, but if it does what you want, or if there is an Add-On out there that does what you want, then its simple and easy to get going. A bit Plus/Minus of GP compared to AX and NAV, is that its quite rigid in its method of use. Accountants love it because it force the user to press the buttons in the right order, and enter the numbers in the correct sequence. I.e. it allows the users to go into "brain off mode", which is damn hard to do in Navision.
Navision is all about having it your way. the balance between getting down to deep programming, and getting a reasonable level of customization is about spot on.
An interesting side fact, back some years ago, Navision UK did a survey of its users, and had them fill in a number of questions. The most interesting response, was that to the question "what do you see as the major strength of Navision" was "its flexibility". But in response to the question "what is the major problem you see with using Navision" the answer was "its flexibility". The problem is that when people have flexibility and choice, they use it. Its very easy to make changes, so changes get made.
As to Axapta, its a great product, not too different to Navision, but it has one major advantage. And that is the marketing. This means that its much easier to sell big Axapta systems than big Navision systems."