Keep in mind, Baba, that radioiodine therapy is a well-established treatment that has been around for quite a long time. Sure, nothing's guaranteed these days, but the safety and effectiveness are well known, and are certainly less than the disease itself.
Though it isn't my field of expertise, I dealt with a number of patients who had the I-131 treatment, and don't recall any major problems.
Personally, if all was equal, I'd choose the cocktail over the surgery.
I would follow the advice of your endocrinologist regarding your specific case.