Garmin is just currently the leader in running and cycling computers right now, so they are often recommended. The Edge 500 is hands down the most popular high end cycling computer at the moment, but it runs nearly $350 once you get a HRM and a cadence sensor. The FR70 is their super low-end watch, which I listed just for a budget option. My wife uses our FR70 on bike rides just to measure HRM, and we got the speed/cadence sensor for $35 so we can also measure how fast she's going. The data files are nice to look at after the ride.
Another plus side to garmin is that they're all ANT+ compatible. ANT+ is the current wireless standard used by most heart rate straps and wireless speed/cadence sensors. Other companies like Polar may use a different tech. There are some thoughts that bluetooth may take over this market at some point, but for now ANT+ is the leader.