You might consider posting a signature with your hubbty's stats, e.g. Gleason, stage, age, PSA, etc...that will help members give you their best advice.
I understand your husband's concerns with ED, I'm sure all of us on this forum have that concern. But, for me and I think most men I have spoken to re quality of life, the priorities are 1. Cure the cancer, 2. Normal urinary functions and 3. ED...in that order.
The other thing that I would strongly recommend is that you and your husband do lots of research and get multiple opinions. Depending on his "stats" there are probably many good options for cure. At the very least you should speak with an experienced (250+ procedure) robotic surgeon and an experienced radiation oncologist.
If you are interested in a seed journey, you can go to "Tudpock's Brachytherapy Journey" which I just updated since I had my procedure yesterday.
Good luck and let us know how you progress.