Hi Mark,
You have a similar choice that I was faced with, prostatectomy which would probably not be nerve sparing, or radiotherapy. In my case, they thought it likely that if I had a prostatectomy, I'd have to have radiotherapy afterwards too.
You are correct that the PDE5 inhibitor pills (Cialis, Viagra, etc) will not work if you don't have working nerves. These drugs effectively amplify the nerve signal in the penis, but amplifying a zero signal will still give zero. They help if the signal arriving is weak due to partial nerve sparing, or only partial nerve recovery so far, or low signal due to low libido/arousal.
So two things swayed it for me.
1. Sexual function was important, and radiotherapy offered a much better chance of that still working afterwards when nerve sparing unlikely.
2. If I had a prostatectomy, I'd likely end up with both treatments and both sets of side effects, so why not just go with radiotherapy? (May not apply to you - I don't know your precise diagnosis.)
If you go the radiotherapy route, things to consider:
You will probably be on hormone therapy for a time, and for 80% of men, that switches off libido and sexual function. That's temporary just while you're on hormone therapy (plus a recovery time afterwards). However, if you don't put in regular effort with penile physio, you will likely get to the end of the hormone therapy, get your libido back, but find your penis doesn't work anymore. 80% of men on hormone therapy can't get erections, so need to use a pump daily or at least every other day to have an erection. There is some weak evidence that taking 5mg daily Tadalafil will also help by improving blood circulation in the penis when flaccid (and if you're one of the 20% who can get erections on hormone therapy, it might help with that too).
There is still a chance of radiotherapy damaging erections, but this is lower than with nerve sparing prostatectomy (and much lower than with non-nerve sparing prostatectomy). If this is going to happen, it happens slowly with the first signs by 2 years after radiotherapy and a gradual deterioration continuing for years. The main cause of this is fibrosis of the blood vessels feeding the penis.
I am now over 2 years past the radiotherapy, and hormone therapy is wearing off. I am lucky to be in the 20% who could get erections on hormone therapy, and that all still seems to work although I should have used the pump more than I did earlier on. If you have any questions, do ask.