Video quality on 1080p Blu-ray and 4K Blu-ray is usually significantly better than 1080p streaming and 4K streaming respectively. A lot of times 4K streaming looks like well upscaled/post-processed 1080p Blu-ray video. There will not be as much of a difference for some titles if the disc is heavily compressed to fit more episodes on it.
As for audio, you may or may not notice an audible or obvious difference between streaming or disc, but streaming is almost never lossless and sometimes dynamically compressed under the assumption it will be consumed in a nosier more casual environment.
Personally, if there is something I care about I will get the disc. If it's more casual viewing then I will stream it.
