I'd really consider an elemental shaman for a caster group.
They'll bring a lot more then a mage to a boxed group.
Tremor is nice too, for certain encounters.
Heroism is pretty huge at times too.

If you're willing to manage the Eclipse procs.
Or have some kind of macro that does something towards that end.
A balance druid is also huge for a caster group.

Personally, I'd go for one of each, and scrap both mages.
I like the utility the mages bring, but both the shammy and druid bring stronger dps and are easier to box.
MotW > AI, even for a caster group.
I do like the portals on the mages, but not enough to take a mage over either boomkin or elemental.