As @jeffmarc rightly points out, you have lots of corroboration that the cancer is real and needs treatment, but I hope it's reassuring that Gleason 3+4 is the absolute lowest result that oncologists are even willing to call "cancer", so you caught this as early as you could have, and your prospects are excellent. Both SBRT and surgery are equally-effective treatments for patients in your situation, though neither comes with a 100% iron-clad guarantee, of course: cancer is a tricky little devil.