Originally Posted by zoenphlux
With all that being said, what would be the wisest thing to continue collecting?
A good range of 4K Ultra-HD Blu-Rays are actually from 2K+ (FHD) masters that have been upscaled and expanded to HDR so in some cases the improvement over FHD is not massively significant. If it's older 35mm+ film footage it usually should be 4K if it's been scanned in properly, however then you have to watch keeping in mind the technology used at the time to record the film can impact the quality of the 4K output. Check https://4kmedia.org/real-or-fake-4k/
as a potential reference (there used to be RealOrFake4K but that's gone offline). Digital copy wise the Apple TV 4K seems preferable since they will release the movie in 4K and optionally Dolby Vision/Atmos automatically if there is an Ultra-HD release, otherwise best to move to a service that provides 4K streaming like Netflix if you aren't into collecting. The one thing Ultra-HD has usually over 3D/2D Blu-Ray is Dolby Atmos audio, so if you wanted that it's usually locked to 4K only.
If you like 3D then definitely it is worth collecting as the 3D Blu-Ray disc is generally the only way to get maximum quality - I don't think streaming services will continue to support 3D streaming in the long term unless it's in Side-by-Side (SBS) format which would halve your horizontal resolution over a proper disc in addition to having significantly less bitrate. In effect 3D (and 3D Blu-Ray) is winding down (less releases in theatres and then less releases of discs) so from this point on there will be less and less releases to collect, so whatever you get now most likely won't be available in the future except second hand (and you'd probably need to bid vs other 3D enthusiasts at the time). I'd use CinemaBlend 3D
to evaluate the reviewed 3D quality of a film before buying it, the vast majority are post-converted 3D (with the occasional true 3D movie), you can also check RealorFake3D
So pretty much in summary:
4K - Collect only if you wanted it since streaming services will most likely either have it or pick it up in the future. Just need to check it fits your criteria of course.
3D - Collect it if you wanted to enjoy personally or share the experience with others in the future. You won't likely be able to do this again easily.
FHD - Collect it if you wanted it and you know it's not likely to get a proper 4K release in the future (or not a factor).
Out of these I'm pretty sure we're going to see re-releases of 4K/FHD discs over time so there's no super compelling reason to immediately buy them to collect unless they were in some kind of special/limited edition. If you can get by with streaming that's probably a better choice overall, particularly if it's just for yourself.