Jeff makes a good point. Either of the two lenses you've mentioned would be better than good, but I don't think that a replacement lens is the solution to your problem unless the lens you are currently using is damaged. The 50 mm f/4 El-Nikkor is not the best enlarging lens around, but all things considered it's a heck of a lot better than some of the junk I've seen passing for good lenses. I use one on an Omega B-600 for prints up to about 4x to 6x from 35 mm negatives, and it's pretty good. I can't see any difference between that lens and the better f/2.8 version of the lens that I use on my bigger machine. For bigger enlargements, the f/2.8 lens is clearly better.
So what's the problem? Maybe the enlarger is not properly aligned? Maybe you have a vibration problem? Do you use a grain focuser? Getting sharp focus on the baseboard can be tricky without an aid, especially if you're a little older and need reading glasses.
My advice? See about addressing any other potential problems first, and see if things don't improve. Until you've eliminated all possible other factors, the likelyhood of a new lens improving things is not good. Then move on to another lens, if you think it is necessary.