I've heard of the 10% loss anecdote. I've never seen it proven with anything factual though. :/ Infact, everything I have seen and read shows a better picture.
Ethanol has 66% of the energy content. You mentioned both E15 and E10 in your post though so it's difficult to tell which you mean. I tried to find a good E10-E0 price comparison and the best(more recent) I could find had a 10cent spread between E10 and E20 at a blender pump which was about 10 cents $3.29 vs $3.39. Which makes the cost the same per BTU between the two. If you have a 10 cent spread between E10 and E0, it's easy to extrapolate that E15 would have a linear spread and would still achieve the same cost.
Then the question becomes, at the same price per BTU, which would you support? Myself, I have a modest interest in engine performance, as in power of course.
So I'm leaning towards Ethanol unless it costs more per energy unit than Premium than I'd probably buy Gasoline instead.