Ed:(posted 8500 days ago)Some developers optimize a film's degree of sensitivity better than do other developers. I always like to say that its not that some developers give more film speed, but, rather, that some give less film speed. The basic speed rating of a film is determined by the film's emulsion formula. Quite a few of the basic developers (ID11 or D76) have a hard time managing to coax even the basic speed rating out of a film. Some of the developers available will cause the film to lose as much as a stop worth of speed (Microdol X is a fine example here). A few developers claim to increase film speeds, but that is questionable. Xtol claims to give an increase in speed of about 1/3 stop (that's not enough justify bragging about).
With most developers, if there is a gain in one area, there will be an equivalent or greater loss in another area. Those that give highest acutance (edge sharpness) usually also have the most apparent grain (HC110, Ilfoted HC, or Rodinal). Those that give the least noticable grain usually suffer from a loss in edge sharpness and film speed (Microdol X is a fine example here). Developers like D76 or ID11 try to achieve a happy compromise of sharpness and apparent grain. Xtol claims to have achieved the D76/ID11 compromise while actually obtaining full film speed.
I would recommend starting with Delta 400 rated at its nominal film speed. Use one of the basic developers like ID11 or D76 and get your film exposure/development worked out so that you get the best negs you feel you can. If more shadow detail is necessary, lower the speed rating you use, and adjust your development times to give negatives that have the appropriate contrast for easy printing. Then, once you have established a good base for comparison, feel free to try the others and see what they have to offer.