I use blockade because I don't have to aim it, and it actually makes the wall about as large as a Path of Darkness. Plus it looks badass.

looking at the tooltip it doesn't look like you have to target the ground for either one.

I think Lynnessa is referring to the way you have to orient yourself in such a way as to hit the most enemies...

At any rate, I went blockade, but wish I had done the other morph as the extra duration doesn't seem to do much and even larger it still seems hard to get mobs caught in the effect, I have a feeling the aoe damage from unstable would actually be a better choice.

If respec wasn't so expensive, I would have already switched to try unstable.

I chose the Unstable Wall of Elements as it seemed a faster way to put out more damage. I can't really say for sure if I really prefer it as I haven't tried the Elemental Blockade, but I do like how the Unstable Wall goes boom at the end, so that's a plus.

If it helps you to decide one way or the other, @Clarebear, I read a post here on the forums from a fellow that said he tested the way it works with some damage display addon and multiple casts of the Skill do not stack the damage in the same area.