When using a speed sensor, the Garmins will use that data in place of GPS to generate speed and distance (plus all related metrics) as well as the final tracked data that gets uploaded.

Hard to recommend a Garmin over a Wahoo with smartphone. The Wahoo is essentially grabbing data off the Wahoo app on the phone, so uses the phone extensively (if I understand the function correctly). The idea is to get a head unit that can use the in-place smartphone capabilities, without having to mount a $600 iPhone on the handlebar. My biggest complaint about the Elemnt is it's $300 and doesn't offer a color screen, which I find useful on my Garmin 810. Garmin software can be funky as well and they are less helpful fixing many known issues.

The Garmin 520 is not a mapping or turn-by-turn unit. It can do it, sort of, but not the same way as an 800/810/820/1000, which have built in maps and offer better functionality for navigation.

All that said, I opted for an 810 in place of using my iPhone 6S on the h-bar. For one thing I would have wanted a protective case like a Lifeproof, which is $100 and I hate the Lifeproof or Otterbox Defender for using the phone off the bike. As well the screens on an iPhone didn't work with gloves (Garmin touch screens do). And I don't want the phone battery running down on a ride and then having to charge it during the day. I don't have the battery issues with my 6S that QC8 has and think he has a bad phone.

My 810 is a nice unit and was well worth the $279 I paid in April. It's crashed 3 times but all fixable and I know why and how to avoid. I do a lot or pre-made courses in Connect, and do TBT routes. It works well. The after the ride save via BT to Connect has been flawless for me in 6 mos. of extensive use, as has the ability to create a course, then BT port to the unit, all on a unit that had known issues, so I've been lucky and happy with the unit.