No need to over-complicate things...
Residency selection works like a free market. The most competitive specialties are the best based on all of the important decision-making factors including future prospects. Although things could change, major market fluctuations are gradual and happen over decades rather than years.
To simplify your decision-making, start with the most competitive specialties and start weeding them out based on your own preferences and idiosyncracies. It is just a process of elimination and then a cost-benefit analysis to settle any ties.
Rads is in a very good position because it is both innovative and well-differentiated. To compare it to its brother specialty, as it were, rad onc is in a metastable position because while innovative, it is not well-differentiated at all and a single major cut to their primary modality would be disastrous. The "innovative" part is less well understood, but the specialties with the highest IQs, most resources, and most industrial involvement will likely be more innovative.
It is also important to consider that any specialty with a small number of doctors will have limited political clout (e.g., derm, ophtho, ENT, or rad onc, among others) but at the same time, will not be a major target for reimbursement reductions. Still, this latter consideration is not a major factor since the MedPAC (or w/e it's called) works on a percentage basis...so if dermies are 1% of docs and derm costs grow to 4%, they begin cutting reimbursements.