Agreed, if I lived in Europe and was doing your type of travel, I would join OK+. But since Im in the States, its NW for me (until it fully becomes DL).
However, if your travel to the States is close to 50%, I would still consider NW, for US North American free upgrades, generous elite bonus mileage, and ability to upgrade on DL, AS and CO (for the time being) free of charge.
If more then 50% of your travel is within Europe, Id go with OK+.