According to this article I just found, EVs are about 21% efficient well-to-wheel vs 19% for an IC engine running CNG, which is practically a wash in my book - especially if you account for the environmental impact and additional cost of the EV battery pack. Oh, and, according to the article, the well-to-tank efficiency is about half that of the CNG IC vehicle, so I don't think the ten times more efficient stat is correct.
Mechanical Engineering Power -- June 2003, feature, "Gauging Efficiency, Well to Wheel"
Actually, a pretty good article. Unfortunately, it doesn't touch on gasoline engines and I'm pretty sure that gas is more energy-dense than CNG.