Hi Col
I have had a cheap Andrews UW 30mm 80deg eyepiece for about 11 years. When I bought it Andrews sold it for about $70. I've heard it was a rebadged Olivon eyepiece
https://www.amazon.com/Olivon-Super-.../dp/B00CHGMIGY Amazon now sells that Olivon eyepiece for USD129 however I can't guarantee it is the same eyepiece I have. It appears identical on the outside. Andrews no longer stock them, haven't in many years.
I recently purchased a second hand 31mm Nagler. So I have used the UW in more scopes and longer than the Nag.
Just like many eyepieces, their performance is different in different scopes. The Nagler is a stunning eyepiece on practically any instrument. I have not tried it on a fast dob. Reviews claim it deals with coma on a fast dob quite well. Matt's comments confirm this.
The $800 price difference isn't always accompanied by a large performance difference depending upon the instrument used. The UW30 works very well in cassegrains, SCT's, & Maks f9-15, I have an 8"f9 Vixen VC200L, the differences in the two eyepieces are not great through this instrument. These scopes are very forgiving on eyepieces. It also gives excellent views through my ED80 f7.5 again with a small out of focus edge the 20x80mm has a 4 degree F0V and 4mm exit pupil, your Genesis will have a much flatter field than my ED80 even though it's faster, a Petzval is flatter. I'd expect the cheaper eyepiece will perform very well in your scope.
The Nag being more highly corrected, retains sharpness right to the edge of field. However it does this with distortion. In a star field, this distortion is imperceptible. Put the moon or any extended object near the edge of the field and it becomes oval shaped.
The UW30 doesn't distort but loses sharpness at the edge of field. As the f ratio drops (gets faster) and especially in a fast Newt, the Nag is clearly in front.
I would not expect the UW30 to work well on a fast newt. Matt and I could confirm this with a comparison of the eyepieces on his 18" scope one night.
For me, the outer region of an 80+ eyepiece is there to give the sensation of looking through a large window into space. Like most observers, I don't do detailed observing on the edge of the FoV. My acute vision is centre of field, & averted vision. The wide field is there for sensation and as Matt said, to help finding objects. So neither edge distortion nor edge focus bother me in the slightest
Your question was, "is the Nag 31 worth the money" indicating a cost benefit aspect, The Nag 31 is probably the best eyepiece in class and will always outperform the cheaper UW eyepiece as you would expect. In some instruments, the difference is marginal, others great. if money is no object, spend the extra $700-800 and get the N31. If you intend to us a fast newt, get the Nag31. If cost is an issue, look at your use, scope type and consider the trade off between edge focus and cost. Olivon or other ultrawide eyepiece. My comments refer only to the 30mm UW, other focal lengths in the Olivon range may have quite different characteristics.
Joe
so I can find out..
Colin , I have the Celestron Axiom LX 31mm ( Not the Lumios ) and it has been put up against a few 31mm Telemanglers over the years and we have all agreed that the 31m Nagler is better by about 5% , , thats the last 5% at the edge in an f5-6 scope and in a f8-10 they are identical .
but I've been thinning the herd and I have a 30mm Pentax 
