If it can be determined without a reasonable doubt that the cancer is contained within the prostatic capsule then the decision seems to turn on the possible retention of some sort of sexual function.
As between Brachytherapy and Prostatectomies, in general you've more chance of performing sexually following the former than the latter.
As between Prostatectomies, you've got more chance of performing sexually following a nerve sparing (Da Vinci) procedure than a radical procedure.
Problems can arise with the Da Vinci procedure should there be previous scar tissue as from an appendectomy or a nephrectomy/nephro-ureterectomy. Scar tissue makes fine surgery tough work. In such cases it is possible that the surgeon will have to make a theatre decision to convert the nerve sparing procedure into a radical one. An example of this, by his own account, is Andrew Lloyd Webber, who went into a Da Vinci procedure intending to retain sexual function after a nerve sparing operation only on awakening to discover that the finest team in Britain at the London Prostate Centre under the direction of Professor Richard Kirby, had been unable to spare his sexual function.
There is one final small but significant point. The end of a man's capability for sex will often affect his partner's psychological well being and sex life, depending on concepts of old fashioned virtues of faithfulness. To embark upon a course of action that will likely deprive one's partner of fulsome sexual gratification makes a treatment decision, insofar as is possible, a two person one. Involvement of one's partner is an important part of the road to successful recovery from prostate cancer and this is true no matter what the actual physical outcome.