I have the same question.
I already own eight Wahoo sensors so I'll go with the Roam first.
My concern is how the Roam displays heart rate zones while riding. If it isn't acceptable then I'll try the 830.
Either way, I will buy from REI as they have 90 day no question returns on electronics.
I have a friend with an 820 and the battery seems to have gone bad or is out of calibration - he only gets seven hours out of it at this point.
Needing a phone to configure the device is a drawback for me. I'd rather configure the settings on a website and sync the device as Polar does.
-Tim-