I’ve read from lots of people that the original 12.9 Pro (not the 9.7 right?) has no problem at all running it, and takes advantage of all the features (except the 3 multitasking things at once, which no one really knows why).
The only issue I'm having on my iPad Pro 12.9 is that when updating all apps that need it at once I get a message saying that apps couldn't be downloaded because there wasn't enough space. I have 5 GB free. Sometimes if I do one at a time it works but Affinity Photo still won't update. Restarts haven't resolved it.
Upgrade to IOS 10- If you have iPhone 6 or newer, iPad Air or newer.
Upgrade to iOS 11- If you have iPhone 7 Or Newer, iPad Air 2 Or newer. If you don't mind HUGE battery drain then might as well upgrade your iPhone 6/6S to iOS 11 but expect battery to drain worst than before. Only iPhone 7 and newer has some HVVC encoding, to adopt the hardware somehow while 6/6s lacks it.