From a WotLK point of view, I'd go with the Shammy DPS and the Druid Healer.

Totems, especially Totem of Wrath is pretty huge... and you already have the 13% spell power buff from the warlock.
And then you don't have to worry about the Eclipse proc at all.
And a Druid/Priest seem to be the best boxing healers, especially on mobile encounters.

However in Cataclysm... the choice is not so clear cut, at least imo.
The Eclipse system will be changing to something where we cast so many of one and then so many of the other, which will be easier to manage.
Not sure if offensive magic or poison will be the better extra dispel, but I believe they otherwise overlap.
I do like the Earth Shield quite a bit, but Wild Growth is extremely nice too.