It doesn't matter which side of the Severn Crossing it is on, now that they are supposedly doing away with the tolls, but an airport pretty close to either side would do pretty well what with M4, M5 and rail connections.
Alas the subject matter around these parts always seems to turn to a Wales vs England thing, think about Basle and Mulhouse as an example, if the Swiss and the French can share that airport isn't there hope for a Wales/England shared airport yet?