I also use an iPhone running RwGPS (or Cyclemeter, depending on what kind of ride I'm doing). I rely mostly on audio cues over a bone-conduction headset, so the screen is off most of the time. This works for me.
Garmins, especially the Edge series, do have some nice features. There's a special climb screen that appears when you're on a long climb, showing the grade of each pitch. You can add plugins to show metrics that aren't available out of the box. AFAICT, no smartphone app can connect to Garmin's rear-facing radar, which some people love. You can control a smart trainer with a head unit but not with any of the apps that are head-unit equivalents. In theory, a smartphone app could do all these things, and I'm kind of surprised none do, especially custom metrics, which are like adding a column with a new formula to a spreadsheet. But here we are.
Battery life has been a non-issue for me. Partly this is because I've got a dyno hub and USB converter (one could also carry a power bank), but even on a rainy ride when I could not use the phone's charging port, I've ridden 12 hours and still been at 20% on the phone battery. I have a bigger problem keeping my headset charged.
Last edited by adamrice; 08-20-22 at 12:03 PM.