I bought the FR305 (comes with the HR strap) over a year ago to run with, and when I came into a bike late last year, I added the bike kit and the cadence sensor.
The bike kit comes with a mount for your handlebars, and a velcro wristband to clip it into when running. Takes all of about 3 seconds to unclip from the bike and clip onto your wrist.
The cadence sensor is not really necessary, since if the GPS is working you will have speed on the bike from that. I like to have the cadence displayed though, especially since I'm still starting out and my legs don't have that 'feel' for what an appropriate cadence feels like.
~Rob