Here is my experience. Streaming 4K from a good source is about as good as a good quality 1080p blu ray video wise. It just doesn't have the bitrate to be as good as a 4K UHD. I will admit, streaming is convenient and easy and we find ourselves doing it more and more, but it's not the same quality as a good 4K UHD disk. Audio is the biggest negative if you have a high quality audio setup. Streaming 4K does usually have Atmos, but it's the lossy variety, and not near the same quality as a disk.
Some of it depends on your system too - on a 65" TV in a living room you probably won't notice a difference, but in an immersive experience with a high quality audio system (and assume your viewing distance is close enough relative to your screen size) there is a difference.
If it's me and it's a movie I want to own, I'll get it on disk, even keeping an eye on the classifieds here and ebay as you can often find great deals on 4K disks (most have the digital code already used). If it's a movie I'm just going to watch once, I'll stream it.
Also for what it's worth, I've found movies rented from iTunes to have the best quality and be superior to Amazon Prime - I'd be interested to see if others had the same experience.
7.4.4 Theater Room: JVC-RS500, Silver Ticket AT 2.35:1 142”, Onkyo RZ830, Anthem PVA-7, Panasonic UB420, Apple TV 4K, JBL Studio 530’s, Dual Driver VBSS
3.1 Living Room: Samsung 64” F8500 Plasma, Anthem MRX 300, Dynaudio Audience 52’s, Dynaudio Audience 122C, NHT SubOne, Roku Express (2019)