Of course above all other things, it's your dad's ultimate choice. It's his body and his cancer. Must be nice to have such an informed daughter.
With a gleason 6, waiting a few more months for a particular treatment shouldn't pose a high risk, not unless his urologist thinks differently.
With the stats you post for your father, I would be more inclined to go for surgery, and if he prefers the robotic, then go for that. There's a good chance with his stats that the PC is contained to the prostate, and surgery could and should rid him of it.
In the rare event it were to come back in the future, then he would have a full range of radiation options to make a second pass on his prostate bed, minus his prostate of course.
With the surgical approach, the post surgery pathology results will pretty well determine fully the event and spread, if any, of the cancer, as well as the other bits and pieces they will remove.
That is my take on his situation. I am also assuming at his young age, he is well enough in other medical matters to undergo treatment or surgery without complications.
I wish him, and you, the best, and hope he can come to terms in his final choice of treatment. While starting out with a gleason 6 is good, as far as having prostate cancer goes, it is still cancer, and whether it grows slow or fast, it is still growing everyday in his body.
David in SC
Age 56, 56 at DX, PSA 7/7 5.8, 7/8 12.3,9/8 14.5
3rd Biopsy Sept 08: Positive 7 of 7 cores, 40-90%, Gleason 7, 4+3
Open RP surgery 11/14/8, Right nerves spared, 4 days hospital, staples out 11/24/8, 5th cath out on 1/19/9
Post-surgery Pathlogy Report:Gleason 3+4=7, pT2c, 42 grm, tumor 20%, Contained in capsular, clear margins, clear lymph nodes
First PSA Post Surgery 2/9 .05, 6 month on 5/9