Editable mesh is mainly used for exotic shapes and objects. Meshes are composed of triangles, and geometry class has taught us that triangles (or any three points) are flat and will render properly. Editable poly is used for dealing with shapes, polygons if you will. With polys, you introduce space. When you get above three points, you can have some variation along th z axis (if you are using the x and y) that will make it render improperly because that polygon won't be flat. If you ever noticed an edge or vertex shading to black, or black holes in your object, this is your problem.
My advice, work with editable poly, thats what I use. I don't like editable mesh personally. But if you get any of the above errors, switch to editable mesh.
Here are some links that will help out:
http://forums.cgsociety.org/showthread.php?t=3034
http://www.webreference.com/3d/lesson114/