Have you had your antibodies tested - if so, do you have Graves ?
PTU has more risk of liver damage, which appears to be the main reason Carbimazole is preferred except in cases of reaction, or in (at least the early stages of) pregnancy. These are strong drugs, and it may be the doctor has good reasons for not wanting you to be on PTU in the long term, so you need to discuss this with the endo, and ask whether there are specific reasons why you can't give PTU a good chance before resorting to RAI. It will take longer than a couple of weeks, although evidence of reduced thyroid levels will typically be there within 6-8 weeks.
While many people (maybe around 50%, although age appears to be a factor) do relapse, that's still a 50% chance of recovery. Even in the event of relapse, some people successfully remain on maintenance doses of carbi or PTU for many years. The chances of side-effects will remain, and it seems some doctor's believe patients may be better-off not having to worry about these . There is also the question of needing to take the tablets every day, potentially in the longer term. While post-RAI, you would probably become hypo, and need to take thyroid replacement for the rest of your life, the impact of an untreated, or under-medicated hyperthyroid condition is potentially more dangerous, more quickly .
There is more info here btf-thyroid.org/information...