I recently got a 20mm T5, although I haven't had a great chance to evaluate it, its a very nice EP. Apart from the obvious FOV it doesn't lose out too much to the shorter fl Ethos EPs that I own.
With a 13 Ethos and the 20 T5, I don't see a need for anything in between - I had considered maybe getting the 17E but then decided on something in the ~20mm range (either 21E, 20ES or the 20-26 Nags), the 20T5 came up first at the right price so that is the way I went.
I nearly pulled the trigger on the 20ES at the start of the year but hesitated and missed a great price and have regretted it since
