To be honest I tried most of those available on iOS last year and gave up because of either the poor quality of data, the poor intuitive user interface, the difficulty of using on the go and the lack of being able to download the maps (I live in an area of poor 3G connectivity). In the end nothing provided significant advantages over Google Maps even if that also has a frustrating UI.
The other problem is that the GPS chip in a mobile phone has very poor resolution and requires triangulation with cell towers to provide reasonable location. I found this out with an accident where the Phone GPS (extracted from a photo) placed me 200 meters away in the middle of the Mekong.
I have used 2 Garmin devices and attempted to download the OSM maps but the reformatting was too difficult (I could not get one of the dlls to work) and I gave up. I have purchased local maps for the Garmins but the subscription fees were too high. There are only two "general" map makers for dedicated GPS (one used by Garmin and the other owned by TomTom).
I have now changed my approach and use Google Maps to research my route carefully (Google Maps download to device is not yet available in my area) before my ride. I also use a De Lorme InReach Explorer for safety as I travel on my own. It has a map function which will get me out of lost situations but I have yet to try that for mapping and the screen is tiny.
Now that BMW/VW Group/Mercedes have purchased the Nokia Maps (Here) division then that package should improve dramatically but will be unlikely to be free if it does.
For the moment I rely on Google Maps which in 95% of instances is fine. I also like street view in Google Maps and I am constantly amazed at the availability of Street View locations (even to some places which are remote and have no cell phone coverage) but Street View is only available for locations that a car can travel to.
Now that I have swapped the Super Sherpa for the Versys I seldom go on the tracks I used to go on (it can handle reasonably rough roads but is not a true adventure bike) so I am seldom out of Cell phone range and can always backtrack to find a reasonable signal.
In summary none work perfectly, the dedicated GPS units are more user friendly. Just try a couple of apps (Google Maps & Here would be my first preference), accept their limitations and enjoy riding rather that fiddling with your phone
PS. Google Maps is more closely tied with Android so Android gets functionality first and sometimes the functionality on Android Versions of Google Maps is not available on iOS.
PPS. Next phone I am going to try Android - it has been 3 years since I bought a device but working with friends Android phones I still find it messy.
PPS. The screen size really makes a difference. I would love to find a dust proof, vibration proof, waterproof case and mount for my iPad Mini. I would not recommend anything less than 5" - Gloves just make it more difficult.