Great question. I had a difficult time deciding between the two fields (as I imagine a significant number of other people do/did) because I like aspects of both. In the end I decided for radiation oncology for many reasons. A few of the main ones are:
-No inpatients. While I enjoy inpatient medicine, it is extremely time consuming. I'm hoping to stay in academics and run a lab, and I think not having to attend on inpatients will allow me more time to devote to my research and family (call me crazy). To be a good med onc doc, you really have to keep up with all aspects of inpatient medicine. That's challenging and fun, but can be all consuming. In rad onc you also need to be an excellent clinician, but because of the lack of inpatient responsibilities, this is more narrowly defined and manageable in my (admittedly "young") opinion. There is lots to do outside of the hospital, and the responsibilities/opportunities only increase as you get older.
-Cool toys. I love computers, gadgets, machines, etc. In radiation oncology we have some of the coolest in medicine. I also enjoy the radiographic aspects of the field (interpreting films, planning treatments based on anatomic concepts, etc.) And the technologies will only be getting more advanced and interesting. Just stay tuned!
-Research. The field is wide-open for contributions from the biological side (my research interests). Small molecules, antibodies, growth factors, etc. are as revolutionary for radiation oncology as they are for medical oncology. Imagine a world of combined modality therapy with "biologically-based" drugs that are minimally toxic and potentiate radiation effects or kill subsets of tumor cells that are resistant to radiotherapy. We haven't even scratched the surface in this arena.
-The potential of molecular/functional imaging advances to revolutionize treatment planning (both of currently detectable tumors and small, currently unnoticed tumors that could become XRT targets). Also, it is quite likely that with advancements in molecular and radiology-based screening test, more and more tumors (not just breast and prostate) will be detected at early stages where local control is not only important but also potentially curative.
-The mix of clinic, consultation, radiology, treatment planning, tumor boards, and limited procedures.
-The "happiness" of pretty much everyone in the field that I have ever talked to, from the oldest attending down to first year residents. Compare med onc fellows/attendings and senior radiation oncology residents/attendings at your institution. I think you'll find out what I mean.
Anyway, those are my two cents. Both fields are great, and can give you a rewarding career helping sick patients who need you. Which one is right for you depends on many factors, and only you can really decide that. My vote is for radiation oncology, but if that's not for you, then I encourage you to go into med onc. We need as many good cancer doctors as possible.