Good luck with your illness; personally, I'd have the radiation -- several friends who had it years ago are doing quite well. One friend who didn't now has a bone metastasis and soaring PSA.
Tennis should not be an issue. I often play with two guys who had prostatectomy -- both went to Johns Hopkins where "nerve sparing" technique is said to be particularly good. The surgery should have no impact on playing tennis, once you've healed.