I mostly use the iPad mini 2 for as a compact multi-touch controller for Max/MSP patches via Mira. Not bad at all for a 5 year old mobile device, and its Retina display is easy on the eyes. It was a hand-me-down item so I can’t complain, and they are pretty darn cheap used these days.
If I got an OP-Z, I’d use it as interface for that.
But if I wanted to go all in and rely on it as a ‘main instrument’, then I’d be looking for something current like the ‘new’ iPad, which is about half the price as a 10.5" iPad Pro. After a certain point, weight becomes an issue in a hand-held, so the larger iPads don’t really appeal to me at this point.