It's the subject of many a debate, though. In my experience I've come to the conclusion that the RAM is the issue. I upgraded from iPad Air 1 to iPad Air 2 (1GB vs 2GB) and the reloading problems went away. But not just that either - whole apps and games didn't reload as often either, for example I'd be playing my old time favourite game Bloons Tower Defence 5 (Don't judge me haha!) and quit out and did some web browsing and opened a few other news apps etc.
Much to my surprise I went back to Bloons about 20 minutes later and the game was still running, it didn't reload when I went back in (as was often the case with iPad Air 1).
So - in my opinion it's RAM, and the more the better. Either way the 6S
will have 2GB, I'd bet my life savings on it.
As for your iPad Mini 2, don't lose hope! iOS 9 will be a lot more efficient and will likely perform better than it is now despite 1GB