It depends what I'm doing. I try to save time while building stuff, so if I'm trying to do a lot of edge work, it's usually faster to use editable mesh, seeing as how it has some pretty nice edge control. Say I have an edge that needs to be turned... Editable Poly only lets you actively turn hidden edges inside polygons. but if you want to turn visible edges, Editable mesh is the way to go. Plus, rather than having to go in and create your edges, using the create command, and then clicking on one vert, and then the other, you just select the editable mesh's hidden edge, and make it visible. Once you have a good custom keyboard shortcut base to use, then edge work could be about 2-4 times as fast in editable mesh than in editable poly. Especially if you've got a lot of stuff to do. Often times, I find myself constantly switching back and forth from editable poly to editable mesh to get the UI functions I need at the moment.
Best case scenario: I've got two edges, and want to add detail. Editable poly to connect and move the new line, editable mesh to visiblize the new hidden edges, and turn the new edges. and then I'm done. Whereas with poly only, you'd have to connect, then you'd have to go in and manually create all the edges needed... it's basically like reducing clicks and accuracy-dependant vertex selection.
For a while I thought editable meshes were more stable than polys... but I'm still unsure about that one. I just save early and save often, and incrementally save, and export .3ds files and all that good stuff