FWIW, I've managed to collect five 50mm enlarging lenses in a fairly short period of time: a Nikon el Nikkor f/2.8, a Nikon el Nikkor f/4, a Durst Neotaron f/2.8, a Vega-11U f/2.8, and an Industar-96U f/3.5. (These last two are Russian lenses; I was curious about them, and they were dirt cheap on eBay, even shipped from Russia. Actually, I've got three of the Industars; if anybody's curious, I'd be willing to part with one or both of my spares.) I've done some side-by-side tests of these. I raised my enlarger's head to the top of its column, projecting an image that was probably roughly 12x18, although I didn't measure it. I then made 3.5x5 enlargements from the center and one corner, using a negative I'd shot of a test scene that included text of varying sizes in both the center and one corner. I focused using a grain focuser and green light. (I've got a Philips PCS130/150 enlarger with an RGB additive color head.) I evaluated sharpness by judging how legible the text was in the final enlargement. I grant this isn't a scientifically rigorous test, but it was as rigorous as I cared to do.
My conclusion is that my lenses fall in three categories of quality. The el Nikkor f/2.8 is head and shoulders above the others in edge sharpness but not significantly better than the second group in the center. The second group is the Neotaron f/2.8, Vega-11U f/2.8, and Industar-96U f/3.5, all of which are sharp in the center but not quite as sharp in the periphery. In different tests, the ordering within this group has differed, so I'm not sure that there's really a significant difference between these lenses. Dead last is the el Nikkor f/4, which is slightly blurrier than the others in the center, but about the same as the second group in the periphery.
Oddly enough, the sharpness under the grain focuser wasn't a perfect indicator of sharpness in the enlargements. Under my grain focuser, the el Nikkor f/2.8 and Vega-11U are dead even and well ahead of the others, followed by the Industar-96U, closely followed by the Neotaron, with the el Nikkor f/4 trailing badly. In fact, the el Nikkor f/4 was so blurry under the grain focuser that it was difficult to focus; it's possible it suffered a bit in the print results because of this problem. At 8x10 print sizes, these differences are pretty subtle, although the differences between prints made with the two el Nikkors are pretty apparent.
I did tests at both f/5.6 and f/8. All the lenses were sharper at f/8 than at f/5.6, but the difference was greater the further down the list you go (least for the el Nikkor f/2.8, greatest for the el Nikkor f/4). I also noticed some differences in effective brightness. The Vega-11U required shorter exposures than the others, the Industar-96U required longer exposures, and the remaining lenses were pretty consistent between themselves. I don't know if this is just sloppy Russian quality control with respect to aperture markings or if there are differences in how much light the glass lets through.
I also used a light meter to check light falloff from the center to the corner. This varied greatly depending on the aperture. At f/5.6 to f/8, the Industar-96U did the best, with only about a 1/3 stop falloff; most of the others (including the el Nikkor f/2.8) had about a 1/2 stop falloff. At smaller apertures, the el Nikkor f/4 did best, with almost no falloff. At wider apertures, the Neotaron was best, with about 2/3 to one stop falloff (vs. 1-1.5 stop falloff for the others).
One big caveat is that most of my lenses (all but the Vega-11U) are used, and none have been checked for correct assembly, etc. It's possible that one or more lenses is in bad enough shape that a better sample would perform better. Going the other way, if you buy one used it could be in bad enough shape that it won't perform as well as my sample.
As to lens "character," I can't really say, aside from my earlier comment about effective lens brightness and center/edge falloff.
Given the sharpness issues, I'm now using the el Nikkor f/2.8 as my normal enlarging lens. If it ever starts giving me problems, I'll pull out one of the others from the second group. I don't intend to buy more 50mm enlarging lenses, but as they're getting as cheap as popcorn on eBay, it might be hard to resist adding another to my collection. I've seen people wax poetic about Fuji EX and Rodagon lenses.
