(layperson, again)

This study is pretty old now — radiation therapy, surgical techniques, and pharmaceutical treatments have all advanced massively in 20–25 years — but for *localised* prostate cancer (below stage 4), it found no significant difference in 10-year overall survival among patients who received radiation, surgery, or active surveillance.

As far as I know, there's still no major evidence that surgery gives a better survival outcome than radiation at any stage — that's what my onco team (researchers who teach at a med school) told me — but each approach comes with a different collection of potential side-effects, and that's probably where you need to focus. And your partner may end up with radiation after a prostatectomy anyway, since their cancer has already escaped the prostate.