I have been using the iPhone version on my iPad and iPhone for 3 years now, it's awesome. Here r my observations;
Glare and mounting position: - if you have a roof or Bimini top and you pay attention to the angle you mount the ipad you should have no problems with reflections on the screen. When sitting at the helm and looking at the iPad, if the reflection points out the back of the boat or through a window you will have glare when it is bright out. BUT if the reflection points up to the roof of your boat you will not get bad glare or reflections ever. What I mean is if you put a mirror on the screen of your ipad And looked at it while driving your boat you would see your roof in the mirror. If you have no roof or Bimini ipad might not be the best option.
Confusion about the iPhone app being used on the iPad: - I have an iPad 4 with internal GPS. About 80% of the screen is usable with the iPhone app. It is just as clear as the iPad app but smaller. My buddy has the iPad app so we can compare both apps together.
I have used the iPhone app on an iPad mini also and you still only get 80% of the screen usable on the smaller mini screen.
The map adjusts to landscape or portrait on the iPhone app HOWEVER The menu only orients to portrait mode in iPhone app. iPad app adjusts menu to landscape also.
Also the speed display only shows in portrait not in landscape in iPhone app. iPad app shows speed in landscape and portrait.
Both the iPad and iPhone app are good value I think but I had iPhone only initially so got the iPhone app. It looks like we will have an annual renewal cost coming on these apps. iPhone annual renewal is $8.99. I guess iPad app will be more than that.
Never any problems with accuracy of the GPS and the sonarcharts are great. It is easy to toggle between sonarcharts and normal navionics chart. You can see examples of both above.
We have an older lowrance GPS fish finder onboard also. Navionics app is very intuitive and things like navigation and plotting a course is waaaaaay easier than on your normal boat GPS.
I also got the navigation in-app upgrade and some other one-time upgrades that are offered. My 1 year subscription expires in a month and I think I may lose sonarcharts then? This is a subtle change they seem to have made to their business model. Makes sense as how do you make money selling a one-time $20 app to a few boaters.
Hope this helps in your decisions on how to rig your boat.
Cheers