I personally reallllllly want that Nikkor 300mm f/4 Phase Fresnel lens! it is sooooooo compact for a 300mm lens! It is close in size to the Canon 135mm f/2 lens, which is in no way a PITA to carry or use, so it is VERY small for a 300mm lens.
You need to decide: ultimate sharpness and overall image quality OR focal length flexibility with adequate image quality-which has long been the prime versus zoom lens dilemma. Personally, I think getting the right framing, to make "the right picture" is vastly more critical in many types of photography, so a modern zoom lens usually wins out over a prime lens, for most uses, most of the time. For me at least.
However--when the light is marginal, like indoors, or early or late in the day, the wider maximum f/stop that most primes have makes that little bit of extra difference that can or might make a big,big difference; a 300mm f/2.8 lens shot at f/2.8 is a MUCH better a low-light image maker than say an f/5.6 setting at 300mm on a slower zoom lens; sometimes, sheer shutter speed makes a big difference in what types of pictures can be made. That is whwere the 300mm f/2.8 lens, or sometimes even the 300mm f/4 lens, can be better than being stuck at f/5.6.