Depends on your playstyle. Simple as that.
I like to get in the thick of battles and try to hold off 2-3 or more DPS As long as I can. The more that are on me means the less are on our FC or our DPS. Thus I stamina stack.
If all you care about is topping the charts(which I could care less about since I do not have a fragile ego) and standing behind your defenders, then spellpower or a secondary stat may be the choice for you.
/cheers
Sweetsidney