All fixers contain Sodium Sulfite or a Sulfite salt. On the acid side, this releases the very smelly SO2 (Sulfur Dioxide) gas which some people absolutely despise. Some are even allergic to it. On the alkaline side, Ammonium Hypo based fixers release Ammonia gas as the pH rises due to developer carryover (less so if you use a good running water rinse or an acid stop).

So, it depends on what you dislike most! SO2 or NH3... Take your pick. The happy medium is a near neutral pH fix at about 5.5 - 6.8. This does not release gases to any appreciable extent unless mishandled with a poor workflow.

AFAIK, no Kodak or Ilford film or paper needs a hardening fix, all of which are acid and use alum, or which are alkaline and contain formalin and no Ammonia. Recent reports say that some Fuji films may need a hardener. Many European films benefit from a hardening fix. The only way to know for sure is trial and error.

At the present time, I know of no neutral pH hardening fixer.

Any fix containing Ammonia is up to 10x faster than an all Sodium based fixer. Do not add Potassium salts or Calcium salts to a fixer.