I have heard both sides
1. after 1200 Expertise balance your classes key stats
2. Have your expertise as high as you can get it
Hoping to get to clarification
Your not going to get a definate answer on this.... It really does come down to what you prefer. Just switch back and forth between expertise, and power/Endurance mixes to see what works for you. Thats probably going to be the best way for you to find peace on the topic.
To many variables like others have said..Depends on AC abilities, and playstyles you favor as to whats the best combination of stats v. Expertise.
Im not going to even get started on the "Effective HP" discussion, and then you can also consider Power vs. Main stat increase w/Buffs and Percentage increases from Spec etc....Its really a trial and error question once you reach 1200 Expertise its a crap shoot.