[tsugomaru]

I play a priest, so I will try to help you out, although I'm a little out of date.

Level as Shadow.

The very first few talents help you out with mana regen and they decrease down time. Out of all the three talent trees, it does the most damage from 1-80. You'll be able to kill things quicker and with less down time, meaning you will level faster. As far as dungeons go, as long as you don't run any level 70 heroics, you shouldn't need to be holy to heal. I healed all my dungeon runs from 1-80 as shadow. The 1-60 dungeons are a joke. In fact, I was in Shadowform in most of the 1-60 dungeons, switching out only to carry out my healing duties every now and then. The 60-80 dungeons require a lot more focus. You won't be able to switch in and out of shadowform to dps/heal, but it's still not difficult to heal without speccing into the disc or holy tree.

~Tsugomaru