Previously I compared the Strava Android app on my HTC Incredible phone to the Garmin Forerunner 610 on a run I did along the Steven's Creek Trail. I thought the phone might have been slightly better, but it was close.
This time I set off for a city ride without my phone, but I had the Garmin Edge 500 mounted with a strap to my right wrist, and I had the Garmin Forerunner 610 mounted to my left wrist with its integrated strap. I had both set to 1-second sampling.
This wasn't a pure run, as I was also Christmas shopping (books for my nieces). So I stopped in several book stores along the way, as well as a two cellular phone shops (thinking of switching from Verizon to T-Mobile), one bike shop (the black-on-black Specialized SL4 with 2012 Red is very slick), and a chocolate shop (disappointed Girardelli's "sea salt" "Intense Dark" doesn't seem so dark at all from the fat:carbohydrate ratio). Anyway, all of these stops meant loss of signal.
Here's a comparison in the form of animated GIF. Shift-reload if the animation stops and you want to re-view (it loops 50 times before stopping):
Comparison of Forerunner versus Edge data from a "shopping" run on 16 Dec 2012
The Forerunner is the better of the two. It's a fairly dramatic difference. The Forerunner is newer than the Edge, so that may explain some of the difference. There doesn't seem to be any fundamental reason the Forerunner should have more reliable position detection.