The 904 trans, like I have in my Dart Sport is somewhat lighter and therefore has less drag than the 727, but the 727 is bigger and somewhat heavier and sometimes prefered if you plan on abusing it. The 904 is no softie either as some guys prefer it(including some drag racers) as it has a lower set of gears than the 727. If you plan on loads of HP then go with 727 but the 904 is good to a point too.
My car has about 325HP, enough for fun but not too drastic.
