We have to start from where we are.
Filton might well have been a better choice for Bristol's airport, but it wasn't chosen. Similarly, in retrospect, it would probably have been better to divert the river Rhymney and expand the original Cardiff Airport at Wentloog rather than ship everything out to Rhoose. We have to live with those decisions.
The questions now are how do we improve public transport connectivity to both airports and increase air services from both so passengers from both sides of the channel can avoid the awful schlep to the various London airports.
Bristol seems have cornered the bulk of the short haul and budget airline market while Cardiff is making a decent stab at long haul. Personally, I'd give my eye teeth for direct services from either airport to destinations in the Americas and Asia. Naturally, there is competition between the two airports but, in terms of the economies of both south Wales and the West, the emphasis should be on ensuring international connectivity, rather than which of the two airports airlines chose to use.