Based off of your team comp I would personally suggest the Resto Shaman and Shadow Priest route.
Elemental Shamans bring as much raid utlity as a Shadow Priest does but with the Wrath buff no longer being exclusive to the spec it makes sense to go the optimum route for your healers. Restoration Shamans bring so much to the table now compared to Cata that you cannot pass it up.
Ultimately though, the more Shamans you have in your raid the better you will do overall. Stormlash totems do not stack but they can be chained one after another like the old school Heroism days of TBC.
Honestly, bring whichever comp performs the best. Better player can outshine nonoptimum raid comps.
Well we need to find people (we're starting to realize that our old priest isn't coming back for mop like we thought), so I'll go based on what we can find (people with brains can be hard to find) but with a slight preference for rsham. Thanks for the help.