I too am in a very similar position as you. My urologist is also an oncologist and recomended that I have a 50/50 chance of PSA coming back. I saw an Oncologist and he recomended radiation to increase my chances of no PSA return with the margin. I saw a radiation oncologist and as soon as I told him what my urologist/oncologist said he quickly deferred to him. Although he did seem to give the impression that the number and length of the margin(s) was a determining factor.
You dont describe your margin in your post. Mine was a single .5MM I understand. I asked how many cancer cells can live in something like that and he said maybe two to three.
I am seven months post surgical and my PSA's are undetectable so far. My Urologist states after my six month PSA that he would like to draw another one at the one year mark. That to me ( 6 months) is not watching it closely. I am going to request one every three months for a long while. It is bothersome to not be sure if you got it all. I often say if my PSA even blinks I want to do salvage radiation of the prostate bed.
You say your Doc stated that you can not radiate if your PSA comes back. I hope that's wrong for I based my decision on that I can. After a period of time after surgery they call it Salvage Radiation not adjuntive radiation and I understand the latter is more powerful when there is PSA detected so I would assume you have more chance of side effects if it comes back but thats a huge IF.
I have read some posts here where some of us change our minds for the pressure of PSA coming back and not doing all we can weighs heavy on ourminds and that alone effects quality of life.
All I can say is do your research and take action based on who you are and what you feel is right.
I wish you the best in your decision.