Some years ago one of the photography magazines tested enlarging lenses. They tested the EL-Nikkor 50mm f/2.8 (a plasmat) and a whole slew of f/3.5, f/4 (but not the Nikkor), and f/4.5 lenses (Tessars) including Schneider, Rodenstock, and Leitz. At f/5.6 the Nikkor resolved 80 and 65 (center and edge) line pairs/mm whereas the best of the others at f/5.6 was 50 and 40 (the Leitz - most were 45 and 35 or 40.) Test the two f/2.8s against each other and keep the best one if there is a difference. You will have the best 50mm you can get and then you can forget about the lens and work on the other big problems - the parallel alignment between the neg carrier, lens board, and easel and the flatness of your neg. An easy way to get a flat neg is to take a glass (preferably the anti-Newton one) out of a Gepe slide mount and glue it to the top half of your neg holder.