NWA's program is more generous in some ways, but, besides Economy Plus on UA and complimentary domestic upgrades on NW, the biggest difference is SWUs. Northwest and Delta don't have them, so that's a major disadvantage for your international flying. I have never attempted to book an upgrade with mileage on either UA or NW/DL, so I don't know how they compare. UA has always treated me wonderfully me in irregular operations (either mine or the airline's), which has kept me loyal. I don't have much experience with NW on this count, either, but only bad experience (mostly as a non-elite) on DL.

(Actually, I remember a mailing awhile ago where Northwest unveiled their version of Elite Choice, which did give one the chance to earn a couple. But that required some mileage level much higher than the Platinum threshold, so was unrealistic for me.)