See which is best cannot be said by me because each of them have it's own pros and cons, advantages and disadvantages, it's own opportunities and placements. So its you, who have to know where your interest will lie more so that you stay dedicated for the same and give your best in that field.

First, the word “oncologist”  is used to narrate a physician that does mastery in treatment of cancer. A radiation oncologist works alongside surgical oncologists and medical oncologists to determine if and how radiation oncology fits into a patient’s treatment plan.

Refer to this link to get a more detailed information..

https://www.careers360.com/oncology-course

A radiologist specializes in using medical imaging techniques to diagnose and treat different conditions, including cancer. In fact, almost all cancer cases require a radiologist to perform an imaging procedure for the initial diagnosis, whether it be a CT SCAN, MRI SCAN or XRAY.

Refer to this link to get a more detailed information..

https://www.careers360.com/radiology-course

Best wishes. Thank you.