PHONE OR IPAD:
I was going through the exact same thing except just deciding between Iphone 7/8 or ipad 2. Wasn't considering ipad 4 due to the price. I figured even if I got an iphone 7/8, I'd still want the ipad eventually. Also I wanted to fly sooner than later (currently have an iphone 5 that's not compatible with Go4) and wanted to wait for the iphone 8 to decide if I wanted that or just the 7. Or maybe even 6. I heard the iphone 8 might be $1,300! *gasp*. Searched around for ipad prices and I grabbed one at an excellent deal. Still some good deals out there around where I live. Hope the same is for you.
Case:
Are you the type to buy a case for every device? If so, you'll need to figure out what works and whether you'd be okay removing whatever device from its case every time you fly if it's too big.
Storage:
The phone will always be with you. Not much to think about since you probably already do it anyway. Figure out how you'll store the ipad. Hard case, messenger bag, DJI combo bag? Whatever it is, realize you need to carry it around.
For me, I wanted it to fit in the DJI combo bag and learned about the 3 ways to do it. I'm going with one of them but it means that I can't have a thick case. Whatever you decide, just don't forget to think about storage while you're traveling if that's important. Some people fly right out of their trunk so they don't care how heavy or bulky stuff is. Some people move around a lot and are only carrying the DJI combo bag, so carrying it all together might be more important. If you've got kids running around with you, you might not have enough hands to carry a tablet if it fits in your bag. Something to consider.
IPAD:
I could be wrong and maybe someone call me on it if it's not true, but I thought I read that awhile ago the
Mini 4 wasn't compatible. I got the
Mini 2 mostly for the Mavic but of course I still use it for games and shopping apps. 16gb for $130. Deal on craigslist. From what I've seen of asking prices, they're mostly $160-180 for wifi only ones. If you don't have cell network, you can't use the maps and no "target tracking" is what I keep reading.
I'm able to target track fairly easily though, so not sure what the deal is. I had the
Mini 1 already so kind of annoying that I had to buy a new device. If you're looking longer term, buy the
Mini 4. It won't go obsolete so quickly. If you want value immediately, then
Mini 2 is your best bed.