Speaking as an iOS developer, knowing that the Roon app will be Open GL based I would avoid the iPad 3 as that was the first retinal iPad and suffered from some serious graphics performance issues - basically the same (or very similar) GPU as the iPad 2 but 4x the number of pixels to push.
I would expect (But have no details) that their app will be iOS 8.1 (Or possibly iOS 9 - was are very close to WWDC when the new OS is bound to be announced) so I would recommend an iPad Air or iPad Mini 2 (You ca ignore the iPad Mini 1 as it is an A5 ARM processor (The original iPad Mini is basically a shrunk iPad 2))
The iPad Mini 3 is no faster than the iPad Mini 2.
The Air 2 is faster than the Air 1 (& iPad Mini’s) but the original Air is no slouch.
As the Mini 2 is still current you can bet your bottom dollar iOS 9 will be supported & will run well.
PS: If you are ONLY planing on running Roon then don’t worry about storage space (16Gb etc) but if you plan to use for other purposes the 64Gb is a realistic minimum.
Rik
PS: Any reply from Roon overrides all the above.