Run. It's easier, pushes the pulse higher than cycling, more can be done in less time, gives you that elusive endorphin high more often, quicker to get ready for and clean up afterwards, cheaper, more accessible, friendlier, less fuss. Heck, since I can't run no more, I cycle but if I was advised to consider not cycling and thought it might be worth researching, I'd be back risking my knee rather than my prostate.
And you should try saw palmetto. That's reputed to ease any prostatitis.