It's worth mentioning as well that the M203 was designed under the direction of the Army by AAI.
I'm not sure exactly who owns the intellectual property to the TDP currently, but AAI does not seem particularly interested in manufacturing them for commercial sales, and it's a fair bet that unlike the AR15/M16/M4, which were commercial developments first, then adopted by the military, and the TDP is owned by a private concern, and thus under specific licensing and use agreements - the M203 does not seem to have the same restrictions.
Therefore, anyone who has received a contract to build the M203 appears to have access to and be able to use it for commercial sales, similar to the "SOPMOD stock."
Basically, for all intents and purposes, except for specific configuration, all M203s should essentially be identical besides what's stamped on the side, so it's really simply a matter of which you can get cheapest, and which company you like better.
Personally, I'd really love to get an Airtronic M203 if I had a choice!
~Augee