This is an easy fix but also there a problem with the question: Soothing Mist or not.
Soothing mist + Enveloping Mist = 2 sec heal + more mana
Enveloping Mist = 2.5 sec heal + less mana
We always Soothing Mist + Enveloping Mist/Surging Mist in PvP. But in PvE mana really count and sometime Soothing Mist + Enveloping Mist is not really needed.
I think it's safe to say it is ALWAYS better to cast soothing before casting Enveloping mists.
With 5.1 the mana cost of soothing was reduced by 33% and the CHI generation was increased to 35% chance. Which now makes it our primary source of CHI generation and should now be used over jab in almost all situations while raiding.
To hardcast EM = 2 Second Cast time, which means 2 seconds before a heal starts on the tank or raid member, which is the difference between life and death in some situations.
To Soothing Mist cast EM = 1 Second Global to start Soothing, followed by EM, which means in that 1 Second global you get off a tick of soothing mist on the tank/raid member, a 35% chance to generate a CHI, and EM starts on the raid member 1 second sooner than it would if you hardcasted it.
Hope that clears things up. If you're in a situation where you need to cast EM, the healing from Soothing Mist will help anyways, and the mana cost should be a non issue, as it is now our primary CHI generator anyways.