I bought a used iPhone 6S that's dedicated to the Spark. The Sprint iPhone 6S phones are pretty cheap since they are locked to Sprint and can't be switched to other services like other cell provider's phones can (I bought 2 excellent condition phones for $130 each, find them on Swappa, eBay and Amazon), and you can activate them on Tello for free with a pay as you go plan. I only turn on the data when I need it while flying.
I've not had as much luck with Android devices for DJI Go (3 or 4), I prefer to cache video and only the Apple products seem to be able to keep up with caching the video stream consistently. Otherwise you get a jumpy, jerking video screen while in flight. If turning off video caching (or setting the smallest cache size and letting it fill up to stop the caching) cures that, then your phone is too slow to write the video to the cache and is ill suited to the task.
The advantage of using a cellular iPhone or iPad is that it has GPS (WiFi only Apple devices don't have on-board GPS, they use WiFi for location services), and the map can update while outside of WiFi range thru cellular data if needed. There's also the advantage of using Airmap and Verifly to plan the flight and buy insurance if necessary as well on the phone. I also run Litchi on it, as well as DJI Ground Station.
The iPhone 6S fits the Spark controller perfectly, though you could use the Plus size without issue as well. I also use a sun shade hood and a USB cable to connect it to the controller. But I use the WiFi if I plan to use the DJI googles as you need to connect to the phone first to unlock the Spark for regular use (otherwise it comes up in beginner mode with the goggles, limiting how far and how high you can fly).
I use a Sprint iPad Mini (on FreedomPop) with the Phantom 3 4K that I own, though recently picked up an iPad 4 WiFi in excellent condition for $85 that I may start using. Only reason the iPad Mini works with the P3 4K is that it sends a 480 SD video stream back and the iPad Mini just barely keeps with with it (it's an iPad 2 in a smaller form factor, I'm lucky it works at all!). I expect the performance to improve with the iPad 4. Limits on the iPad Mini and iPad 4 are that they won't run DJI Go 4, only DJI Go 3. But they do run Litchi, allowing them to still be used with the Spark or other DJI quads (though video caching has to be turn off on the Mini).
I use an iPad Pro 9.7" tablet with the Phantom 4 Advanced and it works good, though I'm able to use the Spark's iPhone 6S with the P4A without issue as well (and vice versa, though the iPad Pro is too big for the Spark controller and has to be used sitting to the side).
A friend recently picked up used DJI Inspire 1 and was also asking about what tablet to use. I asked him if he planned to use the tablet for other things or just the Inspire, and he planned to use it for other things as well. So I recommended that he get an iPad and that he get the latest model with the most memory that he can afford as that'll work the best and last the longest before becoming obsolete. He picked up a newer iPad Pro and is pretty happy with it.
James