The more classes you have, the more complicated your macro's.
The harder it becomes to get maximum efficiency from your toons.

Also, the more flexibility you have, because you have more options.
And if you can get the macro's working, your team becomes that much stronger.

Going with a 5-box group between two players, that's not really a whole lot of roles to cover.
A traditional one-player 5-box, might be Tank + Heals + 3x One-DPS class.
That's 3 roles for one player.
You could easily go.. Tank + Shaman. Healer + Mages.
Not really a whole lot of clicking to do.

I'd go with the shaman over another mage.
If it was 1 player boxing all 5, then 3x a DPS class is preferable.