Stack power? Stack willpower? In my opinion neither. Choose your crit and alacrity cap and stack power and surge after that. Simply balance your gear in a way which suits you best.
My stat preferences are 40 % crit and 6 % alacrity.
You can't just consider a crit cap by itself as crit chance
is affected independently by contributions from two stats. A preference can be made like your 40%, but how much of that is coming from will power and how much is coming from crit rating matter. As a result, will power is the best thing to stack if you are able to balance your crit rating appropriately.