My iMac is showing its age now, and to be honest I could happily use a 4K 27" iMac as a second TV for netflix etc rather than spending that cash on a 4K TV as i'm quite happy with my TV and its lack of Android nightmarish bugs that the Sony's have this year (the 172 page thread about the bugs and crashing TV's and losing inputs etc is quite scary) or the bugs that Tizen has with Samsungs curved wallet opening gizmos does not inspire confidence. I would like a dumb TV that I can make smarter if anything in the future by plugging in chromecast or Amazon Firetick 2, not an OS that probably wont get updated much over a year old.
iOS9 seems rushed again tbh but all software is now I think, and its caused battery issues for some, but it runs really well on my Air 2 and I'm happy using that, but tbh I was with iOS 8.4.1. The 3D touch on the 6s is really useful and battery life is great, its and made my old 5s look slow and prehistoric. I went with the 128GB version as taking 4K video eats alot of space. Also I can see the bigger screen without my glasses, an added bonus
At 5 years old my iMac has done very well but El Capitan is making it show its age more, and the screen is to small, and missing out on stuff like handover etc because its Bluetooth 2.0 not Bluetooth 4.0 bugs me. I can still sell it for a reasonable amount and then settle back and enjoy the new machine, which will run metal too, whereas the older macs don't. The temptation though to get the 1TB SSD is very high I have to admit over the 2TB Fusion drive.