Join Date: Oct 2000
Location: Munich, Germany
Programs: LH HON, DL FO/MM, Marriott Lifetime Platinum, Accor Lifetime Platinum, Sixt Diamond
Posts: 6,174
Flying Blue: upgrades possible on AF/KL/NW/KQ (not on CO), free US domestic upgrades for elites on NW (not on CO). No free transatlantic upgrade coupons.
SkyMiles: upgrades possible on DL as well as DL-coded but AF operated flights. No upgrades at all on CO/NW. For top-tier members, 6 free transatlantic upgrade coupons per year.
Generally speaking, pick the metal you will be flying on most of the time if your major focus is upgrades.