now (in frost spec) on my dk:
4.1
frost: 5k strength = 10k AP -> 7500 necrotic strike
unholy: 5250 str = 10.5k AP -> 7875 necrotic strike
4.2 (same gear)
unholy 6k strength = 12k AP -> 9000 necrotic strike

factor in extra damage done due to str buff and value of necrotic strike nearly doubles compared to heal