rad therapists treat cancer patients with ionizing radiation, & nuc med techs are diagnostic imagers who produce medical images by using nuclear medicine.
however, many rad therapists can advance into specialized med dosimetrists, & they make treatment plans for the cancer patients instead of treating them. you can also advance to become a practitioner, but that's only happening in the UK & Canada (only Ont. right now but other provinces are following Ont.'s steps, it's supported by the ministry of health & even most physicians because they were the ones who started to assign more responsibilities to the advance trained dosimetrists & created many informal rad therapy practitioners), where they've already phased out college programs & a bachelor in RT is required (U of Toronto also has a master's program in RT)
salary wise, RTs make slightly over 70k on average in the States, while the nuc med techs make about 65k? (less than RTs, but should be in the 65~70k range). however, med dosimetrists make way more than both professions, the avg pay for med dosimetrists is about 90k in the U.S.