The TZ50 is the only one specifically designed for 6 speeds. All the others are designed for 6/7 speeds.
Aside from that there are differences in attachment points. The TX55, TX35, TX75 are all conventional mount rear derailleurs that screw into a derailleur hanger. The TZ50 and FT30 are claw type that mount to the rear axle.
All of them are designed for city use ie city hybrids, mtb styled city bikes, and ebikes and the derailleurs are very reliable when used there. If you want more gears I'd suggest looking at another derailleur like the Alivio, Altus or Acera which are also city bike derailleurs. If you want to go offroad I'd suggest looking at the Deore which is designed for treckking. If you're starting with a Tourney - there's really no point looking at a RD higher than Deore because at that point the frame probably will be the weakest component in the equation anyway.
Last edited by Burton; 01-01-13 at 09:37 AM.