Not to dispute Jim's recommendation of the G Claron. He's right on the mark Th ey are great lenses at truly bargain prices.
However, WRT to the 300mm Nikkor M I offer the following opinion...
At f22, the first 300mm Nikkor we tested (my personal lens bought new several ye ars ago) was almost identical in resolution to the one 305mm G Claron tested (54 , 54, 54 vs. 54, 60, 54). By f32, resolution of either lens will pretty much ma tch the diffraction limit. Most 8x10 shooters I know rarely shoot any wider tha n f22 (f454 seems to be quite common), and even at f16 the Nikkor is 48 lp/mm co rner to corner. I consider that plenty sharp enough for critical enlargement of at least 6x (a 48" x 60" print from 8x10). So, in terms of practical use, I co nsider the two to be pretty equal in terms of resolution at normal working apert ures.
The Nikkor, however, is slightly higher in contrast, being a multicoated 4/3 des ign (6 air:glass interfaces) vs. the single coated 6/4 G Claron (8 air:glass int erfaces). The Nikkor is also considerably smaller and lighter than the G Claron (52mm filters, 270g vs. 67mm filters 460g).
In the end, they are both great lenses. I just didn't want to see the Nikkor ge t unfairly criticized based on the results of our tests. As a 4x5 color landsca pe shooter, I prefer the smaller, lighter, multicoated Nikkor. I generally shoo t it at f22 or there abouts, and have found the results to be wonderful - both i n terms of sharpness and contrast. If I was an 8x10 black and white shooter, I may indeed prefer the G Claron. Like I said, they are both outstanding lenses a nd I think most users would be happy with either.
Finally, I must point out that our tests results are not meant to be universal. We tested a VERY limited number of samples of any lens (in this case 2 Nikkors and one G Claron). Many were older, used lenses with unknown histories. In the end, our results are really only valid for the EXACT lenses we tested. There c an be sample to sample variations between even relatively new lenses, but especi ally between older used samples. We post our results to share what we've learne d, but mostly as fodder to inspire other people to test their own lenses. How g ood, or bad our lenses are, is really only applicable to OUR lenses. Testing a lens (or a camera for ground glass alignment, or a film holder for proper film p osition) is a simple excercise that requires just a little time and effort. Rat her than take our word as gospel, we encourage everyone to test their own equipm ent. We learned a lot in the process, and wouldn't want to deprive anyone else of a similar educational opportunity.