I've had an Edge 500 for several years and am quite pleased with it for cycling. You can use it with a spin bike for HR recording/monitoring (disable auto-stop or the lack of speed sensor or GPS data will pause it). I've been using the 500 for running and it is acceptable but has limitations - no pace display (I can convert speed displayed in my head more or less), seems succeptible to errors in instantaneous displayed speed from arm movement (recorded data is usually fine), and a bit large to wear on arm. I've kludged up a wrist strap from a spare bike mount and a elastic/velcro arm band for MP3 players. Garmin makes a wrist mount (for another of their devices) that fits the 500 that may work better. Garmin has released the 500's successor, so you may be able to pick up one on close out. I am looking at other devices that support running better (size, wrist band, pace display) including indoors (footpod). If I was starting from scratch, the Garmin and competing vendor's triathlon watches (Garmin 310x for example) might be a better choice as it is designed for running & cycling and you can take it swimming (intentionally or otherwise!) as a bonus. The dcrainmaker.com site and blog has a good set of user-oriented reviews and product comparisons.

I am unsatisfied with the phone-based systems. You need a phone and HR strap that can communicate; this may require an additional dongle. The phone is $$$ and easier to damage than a dedicated cyclecomputer/watch. Power consumption, IMO, will be a problem on longer rides. Since I already have a good cyclecomputer, there's not much reason for me to go this route. Your situation may differ from mine. Advantage is immediate upload of ride/run data to logging & social media sites, better screen for navigation, and possibility of real-time tracking if your phone has coverage. In my case, my family are usually the only people who really care how I am doing. A simple text message: "25 miles to go" and "Done!" and the all-important "home in 0:30" suffice.