I have it on both and rarely use the Garmin. The iPhone is so much faster for scrolling and zooming as well as easier to download exactly what you want quickly. Many times, I move my hunt area and just need to find a bar or two of cell service to load a whole other set of maps.
I usually buy 2-3 states every year. Typically, I cringe at the cost and then it ends up being the best spent money. This year, my daughter lined up on the corner of one property and bagged a great buck on the corner of another one. We absolutely wouldn't have pushed the shot without knowing exactly where we were.