MSD gives, on average, roughly 16 spell haste.
IED gives roughly 8 mp5.
The key difference is that when IED procs, the only way the proc is wasted is if you are at full mana. MSD is wasted if you're not chain casting heals (which is why it ends up being semi-useless for healers, the same as the
[Scarab of the Infinite Cycle]).
Healing, in my opinion, is about dependability, rather than lucky streaks/procs that DPS prefer.
Also, hai2u Skyhoof :p