There are differences between developers; but then there are differences between films as well. Picking them out will depend on how familiar you are with how each film reacts to differences in subject contrast, subject matter (fine detail; large areas of continuous tone etc.) and light levels (how much latitude do you have with normal subjects etc.). In my case, I lack that experience, because I imbibed the philosophy that the bigger variables lay outside those of the film (given modern films) and the work was best expended getting to know a limited range. The differences between developers pretty much follow the same lines; like film, you can get fine grain or greater film speed, but with the addition of acutance effects to increase apparent sharpness. PhotoShop's unsharp mask works in the same way as an acutance developer with the same byproduct (grain if a developer, noise/artefacts if PhotoShop).
Developers are traditionally listed as fine grain, speed increasing or acutance. Fine grain usually means lower film speed and less acutance, increased speed means more grain (which can increase apparent sharpness). Acutance usually means increased grain. The best all round performaner is generally reckoned to be ID11/D76, a powder developer that I've used once in the nearly 60 years I've been developing - and that some time in the early 1960s.
The only developer I've used that made a really obvious difference was Acutol (no longer made) compared to Unitol (no longer made - my former standard developer) and even then only really obvious on one set of photographs of a church exterior in contrasty lighting where the stones stood out as if in 3D.
These days I use Agfa Rodinal (bought before it disappeared - it does last even if R09 doesn't) which is acutance but not fine grain. I wouldn't use it if I were using 35mm film though; it's OK on roll film and above at the print sizes I make (usually A3 or A2).
Discerning differences properly is going to require completely standardised processing (time, temperature and agitation); standard subjects (test charts and step wedges); and densitometer and microscope to examine the results. All this is too much like hard work for me, when in practice I get the results that satisfy me without going to so much effort in testing every combination. After all, developer dilution affects the results as well...
My advice would be to try a middle of the road developer with 35mm and avoid acutance or speed increasing ones if fine grain is your aim.
Finally (sorry to be lengthy) if you want to see the differences between Rodinal (Agfa original) with and without additives compared to T Max developer, the very first issue of SIlverprint magazine (and the only one under that title - it changed name to Ag after the first issue) had a review with vastly enlarged sections of fine detail. I've placed the scans
here.
