Not to mention that the boomkin will likely spec into improved MotW (i havent looked at druid talents in a while, this may have changed)

If you bring a mage, you have a place for all your cloth to go.... I personally love the portals.

Ive run heroics with just my 3 mages, and I have run my tank and healer with another boxers 3 shaman, and the biggest difference was the fact that Mages are very bursty, which on heroic fights, seems to work in my favor. In Raids, the shaman often out DPS me on longer fights, most likely due to me having to conserve mana on my mages.

It's definately a tough choice. Why not lvl both and see for yourself?