You can't really go wrong with either lens. Both are pretty light for lenses of such long focal length and will have plenty of coverage for 8x20. The 760mm f14 APO Ronar CL weighs 1190g and the 760mm f11 APO Nikkor weighs 1280g (weights with mounting flange, but without lenscaps). These are both much lighter than the 600mm f9 versions of the same lenses.
I wouldn't worry to much about the difference in max. aperture regarding focusing ease. On lenses this long, the difference between f11 and f14 is barely noticeable. Slight advantage to the f11 APO Nikkor, but if I was choosing between the two, this would not be an overriding concern.
The biggest difference between the two is size. The 760mm f14 APO Ronar is significantly smaller than the 760mm f11 APO Nikkor. It's not that the APO Nikkor is huge (for a lens of this focal length), but the APO Ronar CL is exceptionally compact. The mounting flange for the 760mm f14 APO Ronar CL has 90mm threads and an outside diameter of 121mm - it will easily fit on a Sinar board without modification. The mounting flange for the 760mm f11 APO Nikkor has 110mm threads and is 145mm in diameter. It won't fit on a Sinar board without modifying the flange.
In terms of filter size, the 760mm f14 APO Ronar front accessory size is M67 x 0.75, The front accessory size for the 760mm f11 APO Nikkor is M110 x 1. If you use glass filters, or other threaded accessories, they will be much cheaper and easier to find for the APO Ronar CL.
Really Big Cameras