I used a Garmin Oregon 600 with OnX map chip and my iPhone6 with the OnX hunt app along with my Delorme Inreach SE in Utah for 10 days last year. The OnX hunt app is awesome and I like being able to download maps to use offline, but what I hated was not being able to get maps if I wanted to move to a different area and didn't have cell coverage to get updates. The Garmin had all the unit boundaries on the chip, but didn't have all the aerial topography that I like using, but at least the Garmin was still useable. I also found that pairing my Inreach with my phone to text my wife at home every morning and night and taking pictures really wore down the batteries. The Garmin didn't need a single battery change and was good to go for all 10 days I was there. The Delorme needed recharging once and the iPhone was charged every night with my battery pack (yes, my phone was on airplane mode, but I took a lot of pictures and I like checking up on my wife and kids).
When I got back home to MO, I tested the same thing while deer hunting. Since I had cell reception everywhere I went, the iPhone OnX app was unbeatable. Not even a contest. I planned an entire late-season day hunt to take advantage of a cold winter rain storm so I can spot and stalk/still hunt deer on public ground I never hunted before (and hopefully have the run of the place to myself). I spent 4 hours checking most of the areas I spotted on the OnX app and was able to enter notes on my findings, mark places for stands for next season, mark existing stand locations I found, and places that I may want to set up a blind. I was having so much fun, that I didn't care about jumping 4 bedded deer (for sure 3 does, not sure on the last one) or passing on a 15 yard shot on a doe (big Oak tree obscuring her vitals). I just considered it a very successful scouting trip and one heck of a beautiful muddy and wet hike.
So, in summary, if you are hunting only one or two areas and take the time to download the maps for offline use, the OnX app is awesome. If you buy a OTC Utah any bull and any cow tag and hunt several units over several days like I did, the Garmin OnX chip will likely work best. If you just want to play around with both or have the option of using both since one is none and two is one, go for it. Just be sure to mark your camp on both devices so if one dies, the other can still lead you back home.