There are many MD/PhDs who go into radiology from molecular subspecialties. Residency programs are generally underwhelmed by these applicants because there are already many of them and the links to radiology there are weak. If you have a strong step 1 score and clinical grades, that's fine to get you in, but the PhD in these unrelated areas won't look so special. Physician-scientist jobs in radiology exist, but they're fairly unusual.
Sounds like what you've done already is more medicine related, which will probably help you continue and transition that work.
Whether either specialty is more friendly for a research career is a matter for debate. There's no data, just a lot of speculation. There are A LOT more physician scientists in IM, and so you hear a lot more from them in this forum. Whether that translates to your personal ability to succeed in either specialty is completely unknown. I wouldn't pick based off some speculated job-market of one specialty over another (which is liable to change over the next decade anyway), but instead choose based on your opportunities and interest.