Mighty spellpower or Black Magic

The Black Magic enchant for weapons will be changed with patch 3.3, and will now give the weapon a chance to proc 250 haste rating for 10sec. Could this be an alternativ to Mighty spellpower?

I tested it on the PTR last night and found that it have a 35sec internal cooldown and ~50% proc-chance. This means we can expect it to proc approx. every 35 + 1,5/0,50 = 38,0sec, which result in 250 x 10/38 = 65,8 avg. haste rating.

Using DEP-values at 1,509 for spellpower and 1,432 for haste rating, this gives the following output for the two enchants.

Mighty spellpower:
63 x 1,509 = 95,1 dps

Black Magic:
65,8 x 1,432 = 94,2 dps

The two enchants seems on pair with eachother, but in my opinion a static spell power-buff is prefered over a proc-based haste-buff. I will test it again, when patch 3.3 goes live.