For me:
1. Int't upgrades
2. Domestic upgrades
3. Bonus (100-125%) miles
4. Elite check-in, seat choice, etc.
I have been NW plat elite for several years but could not upgrade any of my int'l flights because most of the coach class fares I purchase are not upgradable. Last year I did the AA plat challage. So far this year I've already gotten 3 AA round trip int'l (NRT) upgrades for 1/9, 2/6 and 3/6 from V class coach fares. Since I enjoy almost 100% domestic upgrades from any fare including award tickets and 125% bonus from NW I plan to also maintain my NW plat elite.
So, for AA its the int'l upgrades and 100% bonus miles and for NW its the domestic upgrades and 125% bonus miles.