I can't remember which one I use, but not all of them will work with the Hy's wonderful Terrain (backyard image) feature that maps the trees and neighboring houses onto the sky. Any projection that does't work with that is dead to me!
I'm currently using the Equirectangular (trying to get used to...), I do like this style of projection, as in all flat.
Oh and I sill have to try the terrain feature from my observatory box...