Honestly, I would pick DL. When you fly AF or KL, you earn 100%; on SU the earning rates are the same as SU's (75% for discounted economy except 25% for one super-discounted economy which is rather rare; 100% for medium economy). But DL's redemption rates -- especially on ST partners -- are often much lower (e.g., DL award on SU to Geneva is 25k; SU charges 40k for it). And if you do make it to 50k MQMs, then your bonus is 100% and not 50% as on SU.