Reliability trumps functionality in my choices. I picked up a manufacturer refurbished 660LM for use. It's been a workhorse for the Garmin line for a decade now. While dated, it just works. Not only does it pick up US based satellites, but WAAS satellites as well.
GPS signals get you to within 30ish feet, WAAS gets you down to about 6 feet of precision. Most of the Garmin line does this though.
Another reason was the price. I got mine at $249.00 while new units generally go for $500+? The LM is lifetime free map updates. And having been through the Honda Map updates with my truck, paying $150 a year for updates is the suck, so be aware when picking your GPS that you aren't getting saddled with a pay-to-update model.

I also have a Samsung S5 phone, but phones generally use A-GPS, GLONASS and BDS for location purposes and require a bit heavier computing resources. GLONASS is essentially a Russian GPS system, MDS is a Chinese GPS system and A-GPS is a data-based GPS system that uses a cellular, Wifi or Ethernet link to a primary GPS server to provide data. You jam all these together and get a pretty accurate location...but at the cost of a bit of your data plan. No WAAS.

I do like features on the 660 as well. Bluetooth a headset, phone and the GPS together, have the phone locked up and make calls with the headset via the GPS. If you want to use the onboard audio system, just run a line from line-out on the 660 to line-in on the bike, now you can hear the GPS through the speakers, or music or blare out your phone conversations...

I also like little funky stuff like the voice prompt software (my Garmin screams out commands as Foghorn Leghorn and a few other characters that crack me up), the trip planning software (easy to set POI's and routes while not riding).