The short answer to your question is that you pretty much always want willpower over any other stat.
There is no DR on bonus damage granted by willpower or power, however because bonus damage is additive, the more damage you are doing the less power increases your damage as a percentage. This acts as a kind of soft DR, because every point of power is a point not in crit, which increases your damage multiplicatively.
There are also other considerations, which have to do with your spec and other itemization: As a madness specced DPS sorc, alacrity does me very little good, it doesn't increase GCD or speed of DoT ticks, so it really only helps with force lightning, which accounts for about 30-40% of my total damage. Because of this I have itemized surge wherever possible (over 500 in almost full lvl 61 gear). Also I have in my tree +30% crit damage to 4 of the 5 spells in my rotation. All this increases the value of crit%, and after doing a fair amount of math, I have figured out that, at least for my spec, the 300 crit thing is a myth, and I am gaining better returns on total damage/point from crit than from power even though I am over 500 crit.