John, you are in the exact same position many of us here have been in, and which we found difficult for the same reason you do. There are two options and there isn't an easy way to choose between them - in careful studies they have been found to have indisitinguishable success rates.
But there are a few things to be said. Firstly, that the "average" patient in those studies might not be the same as you, do ask your doctors about your specific clinical situation and whether that might change their likelihoods of success. Second, by definition any study measuring ten year outcomes is doing so for surgery or radiotherapy done more than ten years ago; in that time both surgery and radiotherapy have advanced, with the biggest improvements probably coming in radiotherapy as the latest generation of machines have been introduced. And third, although success rates might be similar the side effects may not be, and as others say above it depends on what is acceptable to you.
Broadly, the advice doctors tend to give is that the younger and fitter you are the better chance you have of good recovery from surgery. But even so a percentage of patients suffer ongoing continence and/or erectile dysfunction. At the same time, the big side effect from radiotherapy is the risk of cancer in a neighbouring tissue (bladder or colon) down the line, making it more risky for those whose age means they may become vulnerable 20+ years later. But as pointed out above, the hormone therapy (androgen deprivation therapy) that routinely accompanies radiotherapy can significantly impact your quality of life for several years. In the end it is always a bit of a gamble.
Do spend some time reading through some of the older threads on this forum, you will get first hand reports of what the various treatments are like (including some variants not always offered at first, like brachytherapy alongside radiotherapy).
Good luck. It is hard at this point, but once you have come to a decision it is easier going through with whichever treatment you decided to favour.