Which particle system is best for trees in a forest? In using the particle system to render trees in a forest, is there any advantage in using the Emitter system over the Hair system given that the render will be done in Cycles?

I have played with both using tree images with alpha on planes as well as 3D geometry and can get similar results with either system in my small tests.

I believe that:


Cycles does not support billboards which could otherwise be used in
the Emitter system.  So a track to constraint system is needed in
either case. 
If using the Emitter system, I have to make sure the
trees are not dynamically popping in and out during animation. 
The Hair system has a special panel for Cycles.  If one is duplicating
objects instead of strands, does it matter?


Hair seems to be the logical choice for grass but is it the best system for trees?  I have seen tutorials for both but perhaps the ones using Emitter are older.
I've always used Emitter to populate a large number of static objects. I don't think hair is needed here.

In the particles panel:


Set the start and end value to -1, and the lifetime to some high value that's longer than your total animation. This avoids any popping in and out.
Turn off particles newtonian physics completely.
Use object instancing or group instancing to replace each particle with a tree
Play with other settings (variable size, initial rotation, etc)


Cycles is surprisingly good at rendering large number of instanced geometry. So unless performance is really important, rending actual geometry shouldn't be too bad. And lastly, this tutorial might help.