Hi CR125Honda
OK first the caveat. I'm sure I'm not telling you anything when I remark that apple has it's strong fan base and even the individual products each have their own staunch supporters as well so you can often stir up a little controversy along the way.
Although I don't work in the field anymore I am a qualified Telecommunications Technician so know my way around the hardware. I have 12 mobile devices in the household between the Wife the kids and I. The first 11 are all Android devices of one type or another because being the family "tech guy" I'm the one who does all the work keeping them running and configured and updated and I don't like "walled gardens" so there for I'm not a big apple guy. The hardware is fantastic, I just am not a big fan of the operating systems or the stranglehold they have over apps and what you can do. I mention this to show I don't have any agenda to push.
I got into R.P.A.s and eventually certified as a commercial remote pilot and started a business which uses DJI aircraft exclusively. I tried all of the different android devices I have around the place and found each had an issue of one sort or another (mostly app drop outs). I did my research and found for ultimate application reliability really apple is the way to go (I'm leaving Crystal sky out of the discussion here as you are on a budget before anyone says anything). The reasons why apple is the best choice we'll leave out as you're already decided on apple which IMTAO is a good choice for DJI products.
So I decided to do the dirty deed and buy my one and only apple device to operate the business aircraft. Not being an apple guy I had no leaning towards one product over the other. It just had to work and last long enough to be a good investment.
My first impulse was a Mini4 for form factor and size, as usual I did a lot of research and found in a number of forums that over the last year as the Go 4 app was released and over time has included more and more features there has been an increasing number of people experiencing problems running a Mini4. It's a 4 year old Design and runs the A8 processor so it can't be totally unexpected I suppose.
A lot of people who use their Mini4 ONLY for flying apps seems to be getting along OK still, but people who load a lot of applications and use it for other purposes are starting to report issues with frame latency / choppy video and heat related issues. Do a web search by all means. I don't expect you to take my word for it. I wouldn't.
Even if you are only going to use it for flight related apps the fact that there seems to be so little head room left in the device sort of makes me wonder what happens when future releases of the Go 4 app with extra features come on the scene. The Mini 3 (from memory) has an A7 which is even older and would make me more cautious if I wanted to be sure of some longevity.
In any event, I needed total reliability and I didn't want to be buying a device now and buying another one 6 or 10 months down the track again so I bought a 2018 9.7" iPad over the Mini4 and put up with the holders and so forth I had to buy to make it practical, and honestly not being so young I do appreciate the extra screen real estate, the price was roughly the same anyway so that was not the driving factor.
A recent report has it that they expect an updated Mini5 about mid year which I am sure will be really great but it was not around when I needed to buy and I suspect the price when released may be an issue for you since you are on a tight budget.
Even a 5th Gen iPad sports an A9 processor so on that basis alone and for the reasons above if you want a longer service life I see the 5th Generation 9.7 as being your choice.
Regards
Ari