Resolution isn't the only problem. SD resolution (particularly PAL) is quite tolerable, if it's well-encoded from a good source.

DVDs are not well-encoded, and the sources are typically poorer, too.

DVDs store MPEG-2 Part 2 (H.262) video streams. It's an extremely old, inefficient codec. (It was published in 1996! Next month, it'll be 30 years old!) It looks best when the encoder is given a bitrate limit north of 20 megabits per second, but DVD-Video has a hardware limit of 10 Mbps, and that includes the audio and subtitle streams. Most video streams on DVDs get 4-5 Mbps. MPEG-2 also isn't a very good codec; no matter how much bandwidth you get it, it's never really considered to be “transparent” (that is, encoding artifacts are always visible).

If you take a Blu-ray copy of a film (FHD or UHD, doesn't really matter), scale it down to SD resolution, and run it through a good HEVC (H.265) encoder, you'll usually find that a DVD-equivalent encoding takes about a third, maybe a quarter of the space. Or, if you go the other way and let the encode take as much space as the MPEG-2 one on the DVD, you'll almost certainly see an obvious difference, particularly in action scenes.

Starting a physical media collection? Fantastic. Good for you (seriously). But get Blu-rays wherever possible. You'll mostly have to forego the thrift shop, fine, but if you're ever actually going to watch the film, you'll vastly prefer it.

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simpaticoder 2 hours ago | parent [-]

I have both Blu-ray and DVDs and I've found its the content that determines which is good enough. Kids in care not one bit about image quality. Obviously: people still like retro games, too. But then other movies, like anything by Villenueve or Nolan, or Baraka, really want to be on 4K Blu-ray. But kids movies on DVD are perfectly fine, and sitcoms like Community. (Personally I'd pay extra to NOT see Pierce in 4k).

I recently purchased the Firefly Blu-ray and it was an interesting case because it's image quality isn't that much better than the DVD (but definitely better) however it's sound quality was astonishingly better than the DVD. I imagine this has a lot to do with the source material, how it was mastered, etc. I still stream, but I like that I have a core collection that will never disappear without warning, or be edited behind my back (which happens all the time, without notice, especially on YouTube and on Amazon Prime).

	▲	layer8 an hour ago \| parent [-]
		Yes, for older TV titles, the main reason to opt for Blu-ray is the better sound quality. Although DVD supports uncompressed audio (LPCM), that was rarely used outside Japan, and regular stereo audio typically used pretty mediocre compression. When using subtitles, another reason is the higher-resolution fonts.