The film base and gelatine I'd suppose degrade when long emersed.
It is said that the image silver itself will be bleached, leached.
I've not seen a single study confirming this bit of conventional
wisdom. Everyone, including myself, believe it to be true
or may be true.
 Generaly components of an acid enviornment are less prone to
oxidation than those of an alkaline. So an acid fix would be
the choice IF oxidation is a factor and you like to run off
to the gym while your film fixes.
 BTW, I use fix very dilute, one-shot. It's always fresh.     Dan

  Rapid fixer (Ammonium Thiosulfate) will act as a mild
bleach for metallic silver when acid. This is not a problem
with film because of the rather coarse grains but can be a
problem with printing paper, especially warm tone paper, if
film strength fixer is used. The trick is not to leave the
paper in the fixing bath longer than necessary to fix it.
Film strength rapid fixer will fix out most papers within
one minute and many within 30 seconds. It takes several
minutes at the least to produce any bleaching. For warm tone
paper this is usually visible as a cooling of the image
color. The problem does not seem to exist with Sodium
Thiosulfate fixer. Rapid fixer which is neutral or slightly
alkaline does not bleach. Since the acid in fixing baths is
there for the hardener a non hardening bath need not be
acid. If a neutral or alkaline bath is used one must be
careful that no developer is carried over since it will
continue to work in the fixing bath for a time. Rapid fixers
for paper has been encouraged by Ilford. Their archival
processing method for fiber paper is based on the idea that
the most of the fixer takeup in the paper support happens
within the first minute. So, if you can keep fixing time
under a minute, and use a long treatment in sulfite wash
aid, washing time can be reduced and hypo free prints can be
made. The method requires the use of strong rapid fixer for
the very quick fixing times. I am skeptical that this
approach is actually any better than the more conventional
one of using two standard fixing baths for a total fixing
time of around two minutes for most paper and the use of a
sulfite wash aid with about a half hour wash. I am sure the
Ilford method works but it seems to me to create more
difficulties than it solves and seems no more efficient than
the more convntional system.
 Also, while alkaline fixers are popular right now they
have no advantage over a neutral fixing bath. Thiosufate,
either Ammonium or Sodium, works independantly of the
solution pH. Since the binding action of acid fixers is
actually from the hardener, and since a sulfite wash aid
elimates it there isn't much if any advantage of alkaline
fixers over neutral or low acid non-hardening baths provided
a wash aid is used.
 BTW, adding Citric Acid to film strength rapid fixer makes
a good mild bleach to remove dichroic fog. Citric acid is a
sequestering agent for silver. The amount is about 15
grams/liter of working solution.