I prefer it on my GPS while hunting, it's just what I'm used to though and my phone is usually put away. I do have it on my phone as well and do like to look at that while exploring or just want to look at something real quick. The screen and interface is better on the phone, but I don't want do deal with glitches, crashes, or a dead phone while actually hunting.
I have had OnX on my Garmin 64ST for a couple years. I'd rather use my GPS than my phone in the field as it is a bit more ruggedized. But the screen size is quite a bit smaller than my phone. So this year I decided to use my phone and see how much of a difference a larger screen really makes.