I would also suggest comparing contents of _working_
solutions rather than stock solutions.
  Its interesting to make such comparisons if only because
it shows how few real variations in developers there are. As
far as properties of the various developing agents there is
some information in older books on photographic chemistry.
Most of the developers devised between the late 1920s and
about the mid 1950's are Metol and Hydroquinone developers
which vary mostly in their activity. Before about the 1920's
the most popular film developer was Pyrogallic acid. Pyro
developers originated about 1865, previous to that
non-organic developers were used. About 1950 Kendall of
Ilford found a way to make Phenidone economically. It has
increased in popularity every since. About ten years ago
ascorbic acid began to be used in commercial developers
mainly as a replacement for Hydroquinone. Neither Ascorbic
acid or Hydroquinone is very often used as a sole developing
agent, I don't think Ascorbic acid ever is. Hydroquinone is
used as the sole agent in several very high contrast
developers and a couple of old very warm tone developers for
paper.
 There are many thousands of organic compounds which are
developers. Only a very few have ever achieved much
popularity. Metol and Hydroquinone in combination can make
nearly any kind of developer and is quite reliable.
Phenidone and Hydroquinone makes similar developers which
can have some advantages. Both Phenidone and Metol are used
in combination with Hydroquinone because these form what are
called superadditive developers, having more developing
energy or greater capacity than either alone. Ascorbic acid
is used with both Metol and Phenidone where it is desired to
have a more environmentally friendly developer than a
Hydroquinone containing one. The combination of Phenidone
and ascorbic acid, as in Xtol has advantages of fine grain,
good film speed and generally good tonal rendition. Agfa
makes a Phenidone and Ascorbic acid print developer called
Neutol Plus. The older version of Phenidone has been
superceded by more stable forms such as Kodak Dimezone-S but
their photographic properties are similar.
  In the 1930s, when 35mm photography first became popular,
a number of developers claiming to produce extra-fine-grain
were introduced. Most of these were based on para-phenylene
diamine, mostly used in combination with other developers,
most often Glycin. PPD does produce very fine grain but at
the price of a substantial speed loss. When used as a sole
developer the loss can be as great as five stops. When used
with Glycin the speed loss is less but the grain is not as
fine.
  As better film became available the use of this developer
fell by the wayside. However, color developers are all
related to PPD since it reacts with other substances to form
dyes.
  Pyro is still popular among some photographers. Pyro, in
the right kind of formula, produces a stain image which is
in proportion to the silver image. The stain is yellow,
brown, or greenish brown in color and holds back blue light
to which printing paper is sensitive. A Pyro negative will
usually print with greter contrast then it has visually.
About the only other agents used much currently are Glycin,
which can be used to make very warm tone print developers
which have less speed loss than Hydroquinone developers, and
Amidol, a very active developing agent which makes a good
print developer in combination with sodium sulfite. Amidol
tends to produce neutral colored images on many papers. Both
Amidol and Glycin have become very expensive and do not have
definite advantages so are not often used although there are
photographers who swear by them.