Question:Which is most serious, irradiation by exposure to a radioactive material or contamination with a radioactive substance?
Both are serious, both can kill you.
If you are being irradiated by a radioactive substance, you can stop the irradiation by moving away from the source or putting shielding in place.
However, when you have been contaminated with a radioactive substance, you are continually irradiated by that substance and you cells cannot escape the radiation.
So, practically speaking, contamination is more concerning.