Math is a bit off. 150% of 20 is 30, so the multiplier would be 1.3
I did some rough calcs for my assassin earlier and the two feats came out roughly the same offensively. The +20 melee power would come out slightly ahead, but the ethereal feat has more utility and defense. So overall I think they're fairly balanced.
EDIT: As I mentioned in a previous post, I think ethereal needs to be changed so the displacement effect persists for a time after coming out of sneak mode, say ten seconds to match measure the foe. To balance the feat overall, remove some combination of +2 skills, invis guard, or lower the sneak attack damage to +1 for every 3 or 4 hide. Right now, the displacement while sneaking is useless.