There is not much in difference between Radiotherapy and Chemotherapy in the sense, that it depends on your diagnosis, histology, and how you as a patient is doing.
They are both used for treatment of cancer, or in some cases non-cancerous cases.
Radiotherapy and Chemotherapy are different, in that Radiotherapy is localised, and Chemotherapy is systematic- which means it affects the whole body.
Radiotherapy will only affect the area that is being treated. It uses high-energy x-rays to treat a targeted area. Due to its localisation, it can produce less side effects, as it's only affecting a certain area.
Chemotherapy- uses drugs to treat the cancer and surrounding tissues. It can effectively treat more of an area, such a s leukaemia for example as this is blood borne.
Neither one are better than the other, they are both beneficial in their own ways, and as mentioned it depends what your own diagnosis is and other factors. You should also talk to your Oncologist on what would be beneficial for you, and the Oncologist will know what is best for you. As mentioned it determines whether you have just chemotherapy, or just radiotherapy, or both.
From your local Radiotherapist