Spell speed is a horrible stat for healers. If you cancel a cast once, be it dodging an AoE or cancelling a heal because the tank's already full HP, then you've just lost every benefit of having a lot of Spell Speed. Oftentimes as a WHM, I sit around and wait for damage to come as I let Divine Seal Regen do most of the healing and wait for MP to come back. Unless you're casting 100% of the time, Spell Speed isn't doing anything productive. And let's be honest - there's no fight in the game that matters where you don't have to move, and you're not healing 100% of the time because you can't heal more than the tank's maximum HP.
@ OP: Anything without Spell Speed is BiS. WHMs prefer Det usually, but I like to have a few Myth accessories just so i crit more than once in a blue moon. Crit/det is pretty interchangeable as a secondary stat.