Using Ice Nova on your target gets you an additional 55% damage on from your nova (aoe for half + bonus 10% on that)
So using crystal will give you 38% of a nova in your 3-global burst more than using comet storm.
Obviously keep in mind that comet storm splits damage within 4 yards and crystal splits within 8 yards.
Comet storm has a 30s cd, allowing you to drop full burst every minute with a bonus comet inbetween as opposed to crystal's awkward 1.5m cd.
Crystal requires a fair amount of setup, requiring your target to stay in range for 2 globals after you've thrown down crystal
Comet storm can benefit slightly(likely an insignificant amount) from shatter whereas the crystal damage does not.
Frostbolt buff: lol
TL;DR: Blizzard are cruel to put crystal in game and should remove it or buff comet storm.