To avoid latency (delay in on screen display of what the drone camera sees), you need a heavy processor in your tablet/phone. The iPad Air 2 is the only apple tablet with an A8 processeor. iPhone6 also has an A8. iPad Mini's all have a A7 processor (which is not fast enough to deal with the HD video, especially if you have other stuff running on the mini).
In September most likely Apple will come out with a iPad Mini-4, which will get an A8 as well. If you have an iPhone6 and don't want to buy a rather bulky iPad Air 2, i recommend your to fly this summer with the iPhone6 and buy an iPad Mini-4 in September.
I had exactly the same dilemma but choose to buy the iPad Air-2 4G/64GB for the Pilot Controller, since i don't mind that the iPad Air-2 is a bit big (I also use the tablet for other purposes). On the camera controller I have a Flysight Black Pearl 7" HD monitor with sunshade plus an iPhone6 Plus. Display of FBP7" is brighter than any tablet/phone and there is little to no latency with a HDMi monitor. De iPhone6 Plus is set to map view and Monitor displays camera view.