Join Date: Mar 2004
Location: NYC, ROC, HNL, PEN, BKK, KUL, SIN (check out http://my.flightmemory.com/kkua)
Programs: DL Diamond charter member; 2 million miler (45% from NWA, 2MM achieved 5/2018)
Posts: 682
It would really depend on your geography (where you fly, where you reside and how you like to fly).
If you live in Europe, then, go with NW b'cos:
1) NW and KL have better (and more consistent) service to their elites
2) mileage accumulated on NW can be used for KL upgrades
3) domestic upgrades on CO
Go with DL b'cos:
1) you live by their hub (ATL, CVG or SLC)
2) they have mileage upgrades on SQ
3) you fly often in winter and ATL is less prone to snowstorms