I wouldn't worry. Something better always comes along and you can rely on Apple over and above the others to do a great job. Remember when Betamax was the best or LaserDisc or DVD or HD DVD? And then BluRay was the best. Enjoy what works for you. I love my iPhone, MacBook and Apple TV and the Apple experience of buying film work the best for me. The user interface, the reliability, the content availability, the lovely layout of the iTunes Store.
Picture wise, it depends on the size of your TV. Maybe 77" upwards, you'll 'maybe' see a difference in picture quality between an iTunes 4K Dolby Vision stream and a 4K BluRay - that is providing you can buy the film on a 4K BluRay which outside of the US is nearly impossible. But who would want a 77" or upwards TV. To me, that's vulgar. (Even our 55" TV looks damn horrible when it's switched off for 23 hours a day).
Certainly with our 55" LG OLED, the iTunes picture is simply outstanding. It would take a very strange character to want to find fault with it when let's be honest, once the film looks great, you really should be sat with great friends, eating, having fun and simply enjoying the film!
AV guys could argue all day about compression, picture, devices, settings etc but they really are missing the whole - to enjoy the film.
Personally, I question how long 4K BluRay will be around. It accounts for 2% of the film purchase market worldwide. With HMV now in administration here in the UK, physical disc days are well... But then, I knew that when the iPad launched in 2010 without a disc drive.
Over Christmas we've had family & friends staying over and have watched many films and I've come to the conclusion, that watching films is not for me. I can't sit still looking at a TV for long periods - I want to be creating things, outdoors, playing sports. For me, the weekly film is just great. But these guys who argue over AV really are missing out on... life!
Have a look at this video. Here he is comparing the ATV with a £630+ Oppo disc player...