I think it would be great if BPL did a comparison of these, and other double walled shelters. Of course, since a lot of these are interchangeable, you couldn't cover all of the combinations, but I would imagine that you could cover a bunch. You could also just ask various makers to recommend a few ideas and see how they compare. For example, ask Ron Bell for double walled tent ideas and see what he recommends.
One possible advantage of the Echo 1 is that the inner tent may have higher walls on the bottom. I'm not sure of this, but if so, then it would provide a little more protection from breezes. This is one of the advantages of a double walled tent (in that it doesn't need the ventilation down there). This could make it a bit more comfortable.
One thing I wonder about, is the setup of the inner tent on the Echo 1. The pictures on the HMG web site show the lower walls of the inner tent as being vertical. However, in your pictures, Dan, they seem to flair out a bit. This may, actually be an improvement (for the reasons you mentioned) but it might explain the difference in height. It doesn't explain the difference in width, though.