Synergy in the 5th slot

Right now I am looking at:

Prot Pally
Enh Shaman x3
<insert fifth slot>

I was thinking of going shadow priest in the fifth slot. Ideally I would like to have some raid viable toons at 70 and a SP is always wanted on raids (along with shamans).

What is the overall opinion on the synergy of 3 shamans and 1 SP versus just rolling 4 shamans?

Granted, all shamans would make it pretty easy to handle keybindings while adding the SP is another layer of spell tracking.

Thoughts?

(1-20 is still damn painfull too) :(

Currently I have a 64 Shadow Priest as a 3rd with my 70 Prot Pally and 60 Holy Pally.

The combo spells the shadow priest does is time & key press consuming.

Vamp Embrace -> Vamp Touch -> PW:Pain -> (Mana Burn) -> Mind Blast -> Mind Flay (Channel) -> Mind Flay -> Wand.

I will be trying a Holy Priest as DPS and Heal.

Holy Fire -> Smite -> Smite and even Holy Nova combo.

What kind of spell combo / rotation do you do with the 3 Shamans now?

I'm just starting out the 3 shamans, so it's your basic Lighting bolt, shock, searing totem spam.

I just wanted to get a good handle on what I want to run to 70 before I hit 45 and think, "Man, if I only had a _____ as my fifth slot"