I'm not a software developer and am not affiliated with the church development process, just a quality engineer in Oklahoma. Here is my opinion, for what it's worth:
Those who insist on the iOS architecture will be stuck with Apple products and Apple pricing. I'm not poking at Apple, just stating a fact as they control their distribution network and pricing. Android can be used by other manufacturers so there is more competition on pricing. You have a wider range of build/operational quality with Android devices (both hardware and software) where Apple controls their devices and apps to the same standard. The two architectures are different and my experience is changing from one to another (either direction) can be quite frustrating. (Note: There are refurbished Apple devices available online so if one is more comfortable with Apple, that may be lower cost option for them.)
Kindles run a are highly customized Android OS for the purposes Amazon has for their target market - if you will search the forum, Kindles tend to have more problems running Gospel Library and Member Tools (my opinion only). Again, not poking at Kindle - they have a specific market for their devices. Just that their modifications to the OS can cause problems with other Android apps that are not specifically written for Kindle. Kindle does appear to have more options for parental control on the devices and that may be part of the problem with some Android apps running on Kindles - I don't know.
If cost is important, you can get very serviceable devices running nearly stock Android at reasonable prices (my wife upgraded to a lower range new Android 9 phone last year for free and it works perfectly for her). I agree strongly with Russell that you should purchase the most current OS version you can afford if you want to use the device longer before the OS manufacturer abandons it. I have an older tablet that no longer runs Member Tools, so I use my mid-range Android phone for that (Motorola brand - works perfectly for my needs). If I was to replace the tablet, I would look for one running as close to stock Android as possible, the most current version as possible. There are name brand 7-8" tablets with current version Android in the $100 range which is only slightly higher than Kindle.
Due to security requirements in keeping personal data safe, one should expect the church to stop supporting OS versions when the OS manufacturer stops publishing security updates (thus making the OS version at risk for data leaks). This is an OS manufacturer problem, not a church problem. I'm grateful the church cares for the security of my personal data.
Finally, no one should feel they must purchase a device to do their calling or be included in their ward/branch. If you feel pressure to spend money you don't have for a device in order to be included in the ward/branch, you should express your concerns with your bishop/branch president. If he won't listen, discuss it with a member of your stake presidency. This is a pet peeve of mine because it is their responsibility to communicate/include members who can't or don't have the latest tech (or any tech), not the other way around. The Savior never expected anyone to meet Him at His level - he always met them at their level.
