I haven't posted a lot, but thought you should also look into IMRT/HDR treatment. My Rad Onc said they are seeing very good results, especially in more aggressive Geasons. I was a 4+3 and, although this was initially the least appealing choice, it was ultimately my choice for stopping this beast within me. The HDR can place an extremely high dose inside the prostate while sparing the areas that would lead to more side effects. The IMRT, at roughly half the usual IMRT treatment plan, is still more than enough to destroy cancers in the tissues outside the prostate. The calculated Gys in my treatment was in the 92 range as I recall, and was heavily dosed to the area of known tumors, but also in the entire prostate, and the area directly outside the tumor area. The IMRT especially dosed the rest of the potentials. My description may not be exactly as treated as it's been a while since I read the treatment report, but I think its close.
The biggest risk for ED is if one of the needles gets inserted into the path of the nerve bundles and severs a nerve, but they try to minimize that risk. Other side effects of HDR are probably less than seed brachy as the radiation is only inside the prostate and surrounding tissue for a few minutes, not a few months. Of course, there are probably situations where they want the longer period of dosing that you get from seeds, so that would have to be determined.
Anyway, check out HDR with IMRT. I think it was the best choice for me. Actually, it was my urologist who suggested it and reported that it would be his choice if he is ever diagnosed with PCa. My research seemed to indicate a reluctance by most Rad Onc's to use seeds on Gleason 7s or higher. In my case, I think they wanted to get a little wider spray with the IMRT to the outside areas and a higher dose inside the prostate with the HDR.
HDR is not comfortable, but the usual risks of surgery (side effects) are not as present. My HDR lasted 36 hours lying flat on my back in a hospital bed, being transferred to a treatment center for 3 sessions each lasting only a few mintues. I had 19 hollow 10" needles placed into my prostate and surrounding area. Generally, there are not a lot of issues with side effects. In my case, I was driving my motorhome south for the winter only 3 weeks after treatment ended.
Good luck with your treatment plan, whichever way you choose.