I run some numbers...

Lets' say, 150 hide, that'd be 50 SA scaled with MP.
Now with Scion of the Plane of Earth, you get 2d20 acid dmg scaled with SP.

So the numbers are pretty close, SP is easier to increase and not many stuff are immune to acid, while you don't always get SA.
But, you get an additional 20 PRR, which always awesome to have.

It seems you should always go for Earth (or the other elementals if you want MRR).

The other benefits from Ethereal (skills, blur, invis) are ok, nothing more.

Did I miss anything?
Is there any reason to pick ethereal over the elemental scions?