Mini 4. Nothing else. Actually has a good screen, and 2GB of RAM will give that device probably at least 2+ years of longevity over the mini 3. My mini 2 is the same thing as the mini 3--except it lacks Touch ID--and it's horrid with iOS 9. Very jumpy and sluggish, it's impossible to browse the internet and have more than 2-3 tabs open, switching between 2-3 apps is impossible as well without the apps constantly refreshing because the device ran out of RAM. So I lose my place, all my work, etc.
Animations constantly jump and skip and stutter because of lack of RAM. Yes, lack of RAM. Animations are stored in memory once they are performed. So the first time they occur it might be a little laggy, but it will be stored in memory for later, allowing it to be smooth the next time it occurs. For example, on my mini 2, if I go into an app and immediately come back out, it's smooth. If I use a heavier app though for a long period of time, leaving the app will be slow and it will jump over the entire exit animation pretty much. Real jarring and annoying. This would likely not happen as much on the mini 4 because it would be able to keep that animation in memory rather than having to flush it out like the mini 2 which is choking on RAM.
Also, another thought... Why not get the iPad Air 2? It's the same price as the mini 4 but with even more processing power and a larger screen. After owning the iPad mini 2 for almost 3 years now, I feel that it definitely is too small now. Sufficient, but I'd really love to have a larger canvas. I feel the same with my iPhone 6. At first I thought it was absolutely perfect, but I got tired of it. Too small. Really considering buying a 9.7" Pro but I don't feel like spending minimum $600, I'd rather get a 13" rMBP if I'm going to make a large purchase at the moment.