Per the partner charts, the MQM earning rate for both W fares (W,S,A) and discounted business (C,D,I,Z) is 150%, so there should be no difference in MQM earning. There is a difference in redeemable miles class bonus 50% for W and 100% for Discounted Business, and a difference in MQD percentage calculation, 30% for W vs 40% for (C,D,I,Z). One caveat, on the premium fare deals page there was recently an O fare LHR-Asia on Air France, a business class fare that isn't included on the Partner charts and earns no MQM and MQD on Delta. I don't know if there is ever a situation where a buy-up would be into O, but that would be a situation where you'd want to retain earnings at the originally booked fare.