Hi,
With 2 tumours on both sides you're likely to be T2c.
Speaking from a position of ignorance I'd say that SABR isn't a good choice as it's usually used to treat single lesions in remote areas. It's almost the opposite of standard RT that can treat beyond the prostate.
I'd think Brachytherapy would be similar to SABR in that respect although I'd think it better than SABR as it uses embedded seeds.
Gleason 3+4 isn't that bad, I'd not sure what the psa level indicates as over 20 is usually more of a worry and beneath 10 less of a worry. Unless the lesions are very small I'd think full RT would be the choice and not SABR or Brachy.
Also with your family history they might be more cautious. Hormones might be a step too far but should they be more cautious and include it either before or after RT.
Surgery sounds the more sure solution to me if SABR or Brachy are the other options for a case with 2 lesions on both sides. As you say, RT is an option if it fails, and if surgery fails then SABR and Brachy will almost certainly fail as well.
In summary I'd want a full justification for SABR or Brachy and to know it's not just a Consultants choice of the month. Both treatments being fairly new. I think I'd prefer full RT to the area even though you're young if I didn't want surgery. Yes there are Incontinence and ED risks with surgery and some people think that is too much of a risk.
It's a tough decision and I recall having doubts about my choice of surgery almost up to the date of the op. Although I wanted it out and was elated on the day of the op. I particularly like that after surgery your psa is a great measure of how successful it was only 8 weeks after surgery. I was 67 and risk of side effects was a lower consideration.
All the best, Peter