Hi Don
I always really enjoy reading your input both here and on cloudy nights. It's timely that you posted as I finally had the chance to buy the 21 Ethos yesterday. I picked up the last one from Bintel in Sydney.
I re-read the advice given here, and I had the chance to try the 17E and 21E last weekend, and the 21E did seem the more logical choice. I thought the 17E was slightly more comfortable to use, but it was too close to the 13E I already have. I notice a lot of people have both the 17E and 21E, but I'm not sure that I would use the 17E often enough, except for the fact that it felt like a really comfortable eyepiece.
So I spent a few hours with the 21E last night, and it is definitely the wow eyepiece in my line-up. All of Don's observations are vindicated by what I saw. While the 31N is a great eyepiece for max FOV, the 21E had the nice dark background and pinpoint stars which makes it the eyepiece of choice for cruising the Milky Way and more.
Thanks everyone for your input. If these things weren't so expensive we could just go out and buy them all and try them out. Unfortunately at close to $1000 a pop it doesn't work that way.
Cheers