I have just come across details of the All Wales Coast Path which is due to open officially in May 2012. Provisional route maps have been published here.
The Wales Coast Path follows the principle that "It will be as near to the coast as legally and physically
practicable, whilst fully taking into account the needs of health
and safety, land management and conservation." In the case of Anglesey, this results in both paths following the same route, as far as I can see. Not surprising really, and so if you've done one, you've done the other.
This is not the same when we come to consider the North Wales Path or some other established paths.