You can run GoogleMaps in an offline mode currently with Android and there will be a version for iOS soon.
Where The Garmin is better is finding food and gas stations along your route. Google maps has all my stored destinations in it so that is just like a Garmin. For Gas stations, I just use GasBuddy to find the lowest price gas.
If I'm going from Point A to Point B, and I know there will be good cell reception all along the route, I use my iPhone and GoogleMaps. It's very simple and it usually plots the best route. We travel enough places with cell service, though, that Garmin is needed. If there is an iOS version with offline maps, that would be handy. But GoogleMaps still doesn't have some of the conveniences that a dedicated device does. A big one we use, especially when on family trips, is the ability to add in an intermediate destination en route, without having to ditch the original routing.
Both the GoogleMaps and Garmin seem to be somewhat inaccurate in predicting arrival times; GoogleMaps seems to underestimate how long it will take to arrive, while Garmin overestimates the time needed. It's interesting to see how far into the trip I get before the predicted arrival times start to converge.
Probably the biggest factor in favor of Garmin is that, when I'm navigating with Garmin alone, and my phone is in my pocket, I'm much less tempted to check the phone. When my phone is on and navigating, I notice incoming texts, etc., a lot more. I don't check those when I'm driving, but it is distracting nevertheless.
On a side note, Illinois enacted a "no cell phone use" law (unless it's hands-free) and it has to be the most ignored law ever. Makes me crazy. I can't count the times I've had near misses because the other driver was fiddling with a cell phone...