I'm commuting regularly to Paris, and prefer BA to Eurostar because getting to T5 is easier than the Eurostar station in London. This is a simple question of geography - if they'd kept a Eurostar presence at Waterloo it would have been a much closer call.

CDG is monstrous, but the CX lounge (above the Hermes shop) is the best of the bunch. Factor in a good 10 minute walk in CDG from railway RER station to check-in.

ORY is smaller and quicker. The IB Lounge is at one end of the building by the entrance, and BA check in is at the other end, so if you need to check-in bags it's a bit of a hike backwards and forwards. They walk you from the lounge to the gate and queue jump you to the front of the passport and security queues.

Frankly it also depends where you are in Paris. Both airports are on the north-south RER line B, so there's not a lot to choose.

WARNING: Right now the OrlyVal rapid transit from Orly to the RER B station at anthony is CLOSED after 9pm on Monday to Thursday until 31 July, so if you plan to fly into or out of ORY during the closure - don't. I tried the replacement bus, it took ages, quicker to use CDG.