Mr. Bergman,
The 50 is flow rated at 13.2 gpm while the 55 is rated at 8 gpm. Ceramic balls are available, the 50 requires a 13/32 ball and the 55 requires a 11/32 which are both available at Envirospec. I have the 50 myself and for downstreaming the large orifice size coupled with a good injector provides excellent draw rates. I hope this helps.
Danny