I will provide for you, as a sample, MY start point for the answer of this question:

Hi, I'm Boylston. I already have a L70 Shaman and a L70 Paladin (among other characters), and want to start multiboxing. I'm going to start quad-boxing, and possibly 5-boxing later on with one of my L70s by splitting a character off to another account. My goals are:

* I'd like to be competitive in the 5v5 arena if I partner with a friend. This will be how I want to acquire most of their long-term gear.
* I'd like a competitive setup for BGs, because I want to be able to farm honor for those gear rewards and I find some BGs fun. WSG is not my top priority.
* It would be a bonus if I could spend some time in select heroics (possibly with a healer or tank friend) to get badges, but this is not a primary focus.
* I'd like to be able to 4-box some normal instances to grind cash and enchanting mats. It's OK if it's just some instances and not 100% of all 5-man instances. (Not a problem if I'm limited to a place like Steamvaults, or have no chance at Shattered Halls, as an example)

Say, 50% arena, 25% normal instances and cash farming, and 25% BG, in terms of playtime.

I'd prefer keeping all the classes the same, for ease of setup and maintenance, but I am not opposed to running two different classes. I have a pretty high level of understanding about configuring WoW macros and have a desire to learn more if needed.

Would I be better off with 4xShaman or 4xShadow Priests for these goals?

Also, if I did take the option of adding my L70 Paladin to the mix, how effective would 1xPaladin-4xShaman be vs. 1xPaladin-4xPriest be for the PvE part of my goals. Would either or both allow me to do Heroics if I added that 5th character to the mix?

Finally, I am somewhat open to other classes, but I think I've narrowed it down to the Shaman vs. Priest question. Also of note is that I would only need to roll 3xShaman instead of 4xPriest, since I already have a L70 Shaman. Also, I can powerlevel or instance run my guys to make the trip to L70 more bearable.

* * * * *

Now, that's probably a little overkill, but you get my point I hope. For me, the answer was "I tried the Priests for a bit, but ultimately was convinced 4xShaman was a better fit for my goals".