Which Dynamics Platform?

Not an expert, just someone who looked a few of them.

AX - original target was larger size manufacturing companies, began in Europe, competed with Navision on the fringe of the mid/high market.  both introduce graphical interface and easy customization to the market, before this everything was charater based and not very flexible.

NAV - orginal target mid market retail/wholesale distribution companies, too big for quickbooks, mas90 ect but not big enough for Orcle or SAP implemention - began in Europe little penatration in the USA as first

GP - Same basic market as Navision but primary the US market.

SL - Soloman - same market as GP and NAV, began as charater based and evolved alone the same lines as GP

Navision bought AX with goal of combining and try to reach the low end of the high end market such as Oracle and SAP.

Microsoft entered US market with GP purchase and expanded to Europe with Navision/AX purchase

GP & SL are both of USA origin, AX (Axapta, Concorde XAL) & NAV (Navision) were 2 competitors in Europe - and then came Microsoft an` bought them all To be more precise AX and NAV joined before becoming Microsoft Business Solutions.

As from MS, they position NAV for small/medium businesses, AX for medium/large, the last being generally more expensive for the approx alike functionality. Both ex-European systems have their strong & weak points - NAV in comparison to AX lacks Project Management, has clumsy Manufacturing and so-so Resources&Jobs, but NAV IMHO has better analytical possibilities (especially in v4) and Budgeting, besides there is a bazillion of NAV Add-Ons just for every possible need not covered in base version - a catalogue is available on Dynamics site.

There are of course many other differences, not knowing your needs (and country of residence), nobody can tell you what to choose. Can`t speak about GP & SL, lets wait what will forums` US members say.

BTW, they say in foreseeable future (years, of course) all 4 will mix up in one joint product - MS doesn`t want to compete with itself... 

I am biased.
AX is the baby of the set, it was initiated by Navision/Damgaard when they merged.
Implementation for geographically distributed environment on same platform.
Huge scalability (hundreds of users).
Relativley difficult to adapt for business requirements.
Good functionality for project control.

NAV is the fast maturing adolescent.
New Windows version in the early 1990's - has developed and matured since then.
Have seen it used in 1 man/woman/dog business and as local solution in global corporate ERP strategy and most points between all over the world.
Obeys 80/20 rule : 80% business fit out of the box across all functions and 20% delivered by partner\agent.
Easy to adapt for business requirements.
IMHO provides more functionality, more opportunity with less cost and less maintenance thany any other product in the marketplace (including SAP).

SL and GP.
Who needs them?