As you probably are aware, people arent generally offered a choice as to radiation or chemo or surgery as a mode of treatment. Generally they need a combination depending on the disease and the stage. Its not really a la carte treatment in the sence that one treatment universally surplants the other. Many folks-and doctors too- have a misconception about xrt. Well there are many misconceptions but my favorite is "isn't chemo therapy the new way to go?" and "will rad onc still exist in five years?" There is a woeful lack of understanding regarding the need for multidisciplinary treatment. For a doctor and an oncologist in particular, its unforgivable.
However there is a lot of decision making to optimize treatment for a given patient, and there is a lot of clinical growth and research in the field, which is what I like about rad onc. I like clinical research and seeing patients over the long term which is what you get to do. In particular, you will make decisions with regard to need for radiation, options for approach (brachytherapy, xrt, combination, radionuclide, stereotactic, etc). You will make planning decisions (beam angles, normal tissue blocking. You will do small procedures in sarcoma and prostate and GU tumors (mostly). You can contribute to the optimization of these areas in terms of technique and also general approach to patient care in research. That's what I like about it.
In retrospect, I also am very glad I didn't have to do 3 years of IM and then a med onc fellowship. Med onc was what I initially was thinking of but in hindsight, I have to say I am far happier in rad onc that I ever would have been in med onc. I like doing oncology in particular and med onc docs do wind up having to manage a lot of peripheral issues (managing edema etc). And not having to do floor time and the nonsence involved with it is very nice. All these things are perks I realized after the fact however. I enjoy the occasional "general Medicine" issue as it pertains to cancer patients when they come up (ruling out a PE or infection or managing dilantin or side effects) but I dont have to spend an inordinate amount of time doing that.
Folks with an interest in medical physics and radiobiology will have extra reasons to be attracted to rad onc.