If you love Rad Onc and cant see yourself doing anything else, Id go in to Rad Onc. You just have to know that there will be restrictions in terms of geography and where you work, and that might frustrate you. Probably a lower chance you will be unemployed, but it is not impossible. Alos realize if you are boarded in radiation oncology, it is hard to go find another clinical job outside this field.
If you are interested in other fields, want to be more flexible geographically, what you are doing or where you are working, IM residency will offer you all of that.
There is no way to reliably predict the future.