Will Destroyer versus Crit, nowadays

For mages and knights, at low levels the Will Destroyer astral will usually be better than crit + determination, except on world boss.

If all of the following statements are true for you, then you should definitely switch to  crit/determination astrals:

1. I have 3 or 4 gem sockets open on all my gear.

2. I can attain at least a 50% crit rate.

3. I have a good Determination astral (level 6 or above).

If not all of those statements apply, you may still be better off with crit/determination. Most mages and knights above 100k br are switching to crit/determination, but this number is a side effect of stats that are more informative.

The drawbacks of crit/determination include lower defenses. You must gem and refine into crit if you want a high crit rate, and so this means leaving out some defense stats. Crit/determination also uses 2 astral slots, where Will Destroyer uses only one. Still, statistic scaling favors crit/determination at br's of 100k or above, despite all that.