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Mozzie, I have the 13E, and it is an absolute cracker on my telescope. The few times it has been removed from my focuser on group observing nights, and others have tried it on smaller dobs, they too have been very pleased with it.
The 13E is very sharp across the FOV, has excellent light throughput, and takes the Nagler immersion feeling that one step further. On my scope it produces about 190x without Paracorr, and 220x with Paracorr, so it is only on poor seeing nights that it does not get a gurnsey.
I have been following various threads in Cloudy Nights re the Ethos, and it seems that most find one that is nearest the sweet spot for their scope, and will tend to baulk at the cost of the other focal lengths. Having said that, one bloke says his three premium EP's, being the 31T5, 13E and 8E make the perfect kit, and he doesn't need anything else. I would find three Ep's just a little restrictive.
I have worked out that my 13E, with Paracorr, makes about an 11.5mm focal length. Working backwards, if I was to get the 17E, with Paracorr this would become a 14mm EP. As I expect I would use a 17E with Paracorr most of the time, it would double up with the 13E.
I have decided that I will wait for a 20E (fingers crossed), which I will snap up, but I won't get a 17E.
At the shorter focal lengths, at current prices I would find it hard to justify getting the 6E or 8E. Both are about twice what you would pay for a Pentax XW or Nagler at similar focal length. At high power, most people are not so interested in what may lie in the peripheries of the FOV, but each has their own preference. I doubt I will shell out the bucks for a 6E or 8E until our exchange rate approaches parity with the US dollar again (which may be never).
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