Some points to consider before you make your decision:
-uncertain future of radiology: It's true that money/lifestyle is great in radiology now, but no one knows how long that's going to last. Take into account the recent drastic reimbursement cuts, clinician self-referral (especially cardiology and neuro), lack of control over patients, rise of radiologist assistants/PA/NP. By the time we get out, radiologists may not still be making $500,000 just working 4 days a week. Somehow it feels like the ancillary specialties are more vulnerable to reimbursement cuts and job security issues (anesthesia and pathology in the past). On the other hand, there will always be a need for IM (or IM fellowship trained) physicians because of its responsibility for direct patient care.
-Reputation: When most people think of doctors, they usually think of IM, because IM provides direct patient care and thus they have more publicity. It's unfortunate that lots of lay people still don't know what radiologist is, or cannot tell the difference between radiologist vs radiographer vs radiology technician.
-Length of training: Most radiologists do fellowships nowadays, that's 6 years of training time total. Same for IM plus fellowship. When you finish your medical training in 6 years, would you rather sit in dark room reading films? Or you could be a cardiologist, unclogging coronary arteries, care for ICU patients and have patients and their families thanking you for saving their lives? If you like procedures and imaging, cardiology would be a good choice, especially in a few years, cardiac MRI and CT will be prevalent. Cardiology departments will soon have fellowships in those imaging subspecialties.
-Continuity of care: In IM, you see your patients progress clinically, since the day you admit them to hospital until they get discharged; I think being directly involved in this continuity of care, seeing how the meds/intervention you prescribe help your patients improve over several days is rather satisfying. You feel like making a difference in your patients. I guess you can do this in radiology too; but it would be like following the patient's serial CXR or CT for many days--something I don't find as rewarding
-Mode of training: I agree that IM residency is more exhausting than rad; but overall it's still not too bad. The intern year is the worst, but you need to do that for radiology anyway. 2nd year IM you are senior of the ward team and the interns/med students do most of the work; third year is mostly electives and there's no call during those months. If you go to a cushy IM program, schedule isn't bad at all.
-Someone mentioned providing primary care abroad without being board certified in IM. That all depends on personal comfort level. But I have met attendings in clinically oriented specialties that require prelim medicine (ophthalmology, derm, PM & R) and even they feel very uncomfortable managing bread and butter medicine issues. I imagine that would be even harder for most radiologists who don't see patients on regular basis.
-Try to do a long radiology rotation (1-2 months continuously) to see if you like rad; my experience is that lots of people can sit in dark room for 2-3 weeks and enjoy such rotation, but it feels quite different when you do that for 1-2 months.
Riiiiiiiiiiiiiight... 
