As the original author of this thread, I am returning to report on my results. Having developed a few rolls of Delta 100 in 35mm format, I can say that at least with this film, ADOX FX-39 II and Ilfosol 3 are extremely similar. It is tempting to write "interchangeable".
I am struggling to choose the rigth words here to describe the results. Putting speed aside (which I have not tested), it's about sharpness and grain character. Commenting on sharpness, I can juse type: both developers offer high acutance, and I feel we're on the same page. But the online vocabulary never reached appropriate sophistication to accurately describe the grain character. The blog posts and youtubers stick to the meaningless "fine grain" methaphor. The reason is doesn't mean anything is because people use it to describe two completely different/unrelated effects: grain softess and uniformity.
Well... let me try.
Both developers, just like Rodinal, do not soften the grain. In other words the "grain contrast", for the lack of a better term, is high. But the visible uniformity of crystals is a bit better than Rodinal, it's like Rodinal causes a bit of clumping creating a visual impression of rougher grain. I sometimes describe this effect as grain tightness, e.g. there's very little "empty space" around visible crystals. What this means is that up to a certain magnification ratio, images developed in Ilfosol 3 and FX-39 have this lovely texture to them. This is very different look than D76/Xtol that offer soft (not fine) grain.
I have not printed anything yet, but I have a feeling that I'd prefer the look of FX-39 II/Ilfosol 3 for smaller magnification ratios, but as you print larger and stretch the grain, Xtol may be a better way to go.
Just my $0.02 and only applicable to Delta 100. I haven't touched other films for almost a year now.