I literally just went through all this. Best advice I can give you is to first know the operating system differences in regards to the DJI GO app. The feature sets are slightly different between the two.
For example, iOS offers you 2x digital zoom in resolutions below 4K. It offers slightly more on-screen aircraft data and also allows you to change the name of your bird in the event you ever want to change the name. iOS also allows you to do full-screen FPV. I was unable to get that feature to work on android however I am fairly new at this as well. As you may read in various threads, the iOS app does feel a little more polished however it does appear to be more buggy. Some of the firmware issues that people have been reporting, I did not experience with android. I do experience some of them with iOS.
The only feature that I noticed in Android that iOS does not have, is the ability to tilt the gimbal by running your finger up and down the screen. iOS may have this but I could not get it to work. I actually tried it and that's how I found out about fullscreen FPV .
As for the tablets, I've tried the full price range. I started with a Galaxy Tab A and it performed very poorly. Just not enough horsepower in my opinion for the app. I returned that and picked up the Shield K1 which the performance was amazing. I think in Android devices, the K1 is the best overall value... Most power for the price point, integrated HDMI as well as microSD card capability. The death of the K1 for me was screen performance on bright sunny days. Even with a 70° angle sunshield, it was very difficult to use.
In the end I ended up with an iPad Pro 9.7 cellular. One thing to keep in mind about Apple devices is that unless it has a cellular chip, it is not going to have GPS and as others have mentioned, if you want all the features of the Phantom 4, you have to have GPS in your tablet. You do not have to activate the cellular in order for the GPS to work. Regardless of which Apple device you choose, if you choose to go that route, cellular is only about $130 more for the same device. Seeing that external GPS unit is $100 on average, I said what the heck and just went for the cellular device from the get-go even if there is a chance that I may not use any of the features that require the GPS. It's going to be that one time that I really really need it and don't have it that I'm going to be kicking myself in the ***.
I did a lot of research in the iOS devices. The iPad Pro by far had the most vibrant screen and one of the best antiglare ratings. Some reports I read even indicated that the iPad mini 4 was brighter but when I did a side-by-side comparison in the store, I could not discern the difference. The Pro and the Mini 4 are noticeably brighter than the
Air 2.
If you opt to go Android, I would strongly consider the Shield K1. Just keep in mind that on very bright days, it might be a little challenging to use.
Good luck!
~Joe