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I am far from an expert in optics, but I have invested moderately heavily in Ethos EPs for my 20" f5 so I have some knowledge.
My kit consists currently of the 10E, 13E, 17E, 21E and 41 Panoptic. My most used EP is the 17E which gives 150x in my scope and a nice 30' field. The 21 is very nice but I just find the sky just a smidgen less black than in the 17, and as I am mainly a galaxy observer, the 121x power of the 21 is not really enough.
In comparing Ethos and Naglers, in my opinion there is no comparison. The 13E compared to my old 13mm T6 Nagler gives not just a much wider field but the stars are sharper, focus is easier to achieve and although the eye relief is only 3mm more in the Ethos, 15mm as against 12mm, for a glasses wearer that is significant. The 10E in my scope makes a very nice planetary EP giving 254x. In an 18" UC it will give 192x. I found the best views ever I have seen of Saturn with this EP (PM Paddy for his opinion if you want!) at Snake Valley last month, the Cassini Div was like a pencil line, band of colour, the polar storm and even the Encke Div was visible. Of course seeing was exceptional that night and I would not expect to see it like that many times in a year. The one thing I have learnt is that a bigger scope definitely enables more power, no way would I even think of using 254x in the old 12". So good high power EPs are a worthwhile investment.
One thing to consider with the Ethos is that the 6 100deg versions are designed so that a group of 3 will give a 3x increase of actual field size per increase of focal length. So a set of 6mm/10mm/17mm or 8mm/13mm/21mm will work well together.
Malcolm
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