I am a diehard droid user, I have been using Android since v2.1x and with the kids and family I have 12 Android devices from S5s, Galaxy Tab Notes, Galaxy Tab S2, S3, S8 and S9+. Why? because I'm a Comms tech by trade, being doing I/T for years and I like to decide what I can do with my device not be told by the manufacturer. no walled gardens for me.
Then I have 1 IOS device, a 2018 iPad which does all of my flight duties. I never fly on Android.
DJIs GO 4 app works perfectly well with stock reference vanilla android, their own CrystalSky units are Android, very close to reference and they control strictly what other apps you can install, from all reports they are (mostly) reliable. Other than that or rooting your device and loading a stripped back ROM you're not going to get stock reference Android anywhere. Every company overlays a skin of some kind to make their product "stand out" from the crowd and that's where the trouble starts. Samsung's "Touchwiz" interface is known offender for example, that's not to say every Samsung device will be flaky. Some are fairly good, some are terrible. None are perfect.
When the App writers do the DJI app and updates IOS is simple because Apple keep such a stranglehold on the O/S, the hardware and certifying apps for it. Once the App is written or updated and finalised it just works with any Apple device capable of hosting a recent IOS version because it's the same O/S regardless of device.
On the other hand there is no possible way that the App author can make allowances for every interface on every brand or type of device on Android. There are hundreds if not thousands to take into account. They can certify their app for reference android but they have no control over what the device manufacturer does with the O/S or how other loosly controlled apps will inter act.
I have the added complication that I am a certified commercial Remote Pilot and I work with my aircraft in mission critical situations where safety and threat and error management is paramount. I can't afford any avoidable variables.
When I got my first R.P.A. and while I was studying for my certification I tried most of my Android devices. Surely with that many I'd be set right? Well, here I am with an iPad...... some of the droids were not too bad, come couldn't be relied upon for 5 minutes before a drop out occurred. I only have flight related apps installed on the iPad, GO 4 , Litchi, Drone Deploy, Can I fly there, etc and since buying it I have had
exactly zero issues or drop outs. Not one, not ever. As much as I hate apple (even my iMac computers are boot camped and run windows) I can't argue with the figures.
If you DO decide to go the apple route you may want to think a bit on the Mini 4. I almost bought one, I was only getting it to fly and the size was perfect but as it's now my business I do a LOT of reading at a technical level and I had noticed over the last year a small but increasing number of reports of people with Mini iPads suffering from overheating, excessive video lag or other problems. This was almost always with people who used their iPad for everything under the sun as well as flying. Those who just use them for flying seem to do OK although they do report some heating issues.
It does stand to reason, the Mini 4 is 4+ years since it's last upgrade now and rocks the A8 processor of the iPhone 7 era. The DJI GO4 app has evolved a lot in the last year adding many more functions with the release of the newer aircraft and as such it's requirements for system resources has increased quite a bit.
I'll repeat, if you only use it for flying than you should be OK with a Mini 4 as things stand now, the Spark is not exactly the most demanding aircraft in DJI's stable but you do have to ask yourself how much future headroom is in the unit and what if you buiy a Mavic 2 pro in 6 months? I asked myself and after a
lot of thought bought a 2018 9.7" iPad with the A10 fusion chip and then had to buy Lifthor tablet holders and a full harness and the like to compensate for the extra weight etc, After 6 or more hours flying the setup gets heavy but I haven't regretted it.
Also for that matter, an industry insider famous for doing very accurate leaks on upcoming Apple devices has said that there will be a 2019 entry level iPad and finally a Mini 5 released around June/July so you might want to hold off and see.
New iPad mini 'will launch this spring'
As with everything my experiences aren't going to be the same as every body else, Other people will totally disagree and that's all good. I have to be conservative because it's a job and a Droid or a Mini 4 may be perfect for you I can't risk it and if I was only a recreational flyer I'd still think ..... "why risk having to buy twice?"
Regards
Ari