it all depends on what rep range you want to work in. sinse your trying HST right now and starting at 2x15 you will need the
assisted pull-up/dip machine if you are currently able to preform about 5 unassisted chin-ups. ounce your rep range goes down you may want to try your full BW.
you should try to improve everyworkout, but that can be adding slightly more wieght or more reps in any exersice. your not always going to beable to imporove every lift every workout, but if you can improve on at least one you have improved enough.