Availability -> How much time you're willing to and able to put into the project. I think George Mentioned this because the lead artist needs to be able to spent a little more time on this than the other team members.

George also mentioned using c_move in the other project. As we don't have any 2d-Collision Engine to work with, the game itself should be 3d, if I'm not mistaken. Of course, we can still use sprites here, to make it 2d after all, but using 3d collision hulls. In combination with an isometric view, we could make it look like 2d.
Full 3d will be too much work I believe:
http://trine-thegame.com/site/media/screenshots/trine_screenshot_2009_06_knight_mines_hammer.jpg
At least the backdrop should be 2d, the playing level 3d (collision, c_move) and then we can maybe put more 2d objects on a layer infront of that (grass, clouds, a tree).
Sort of like this:
http://www11.picfront.org/token/y4KU/2010/02/08/1757699.jpg
The further away the sprite is from the camera, the more blurred it should be.
(and tell me if this is nonesense. I just think creating so much 3d art work is too much, and this could be an easy way to stylize the game and make it look good nonetheless.)

Another part that's very important to take into account here is that we need to know all the people who'll do 3d work and who'll do 2d work...
As mentioned in the Email, I'll go for 2d, as I'm more experienced with that.