I have played with druid, priest and pala healer.
I find pala by far the best, a factor here is gear too, but his ability to stun a target every 50seconds is very nice when catching a kill target.
BoP is also VERY nice against warriors and hunters, as it removes all incoming dmg and MS/AS.
The priest was also very nice, because it opened up for a rush strategy against the really defencive teams, having fear ward on my main while rushing in, and tremor totem ready to remove fear from the rest of the team.
The druid however, i found less efficient, the ability to spam good mana efficient heals just isnt there, and the druid buff is gimped compared to stam/spirit and kings/bow.
The druid does not take full advantage of heroism like the pally and priest does.
The pala gets 1second flashes that hits for 18-1900 and crits for 27-2900.
The ability to rebuff easily and mana efficient also increases with an all shammy team.
In 5v5 the nukes rule the street, the paladin is by far the best to take the nukes, and protect with BoP.

When it comes to the general arena fight, its bunker down and kill everything upfront. And a pala that can stand still and heal is a god when it comes to mana efficient and viable heals.