Well the mask just came out so your going to more than likely hear from some folks that have not used the mask yet...
The Posidon is very similar in design, one of the designers of the AGA designed the mask for Posidon. Some differences from the AGA, the air comes over the right shoulder instead of the AGA left shoulder configuration, the regulator is a standard reg and parts will be available more places, the mask is available in two sizes. ( a small percentage of divers have difficulty getting a proper seal with the AGA and the posidon is offered in a small version,
That is about all I can think of off the top of my head, check out the Posidon website, they have a information sheet you can print off on the mask.
If your near a shop that offers them go and try one out for fit,
Hope this was some help...
Jeff Lane