erniescar69
This is a really broad topic with a lot of "ifs" and "but's"
Short answer. The large original file is the studio file. They also provide smaller 'original' files. It's easy to recognize them because they have the word original in the file-name. (Other than SLR Original)
The others are transcoded for the purpose to be compatible with the device they are listed under. Not every device can play the same coding settings.
In theory there always is loss of quality during compression, so also during re-coding. In practice, visually this isn't necessarily the case as much still depends on your hardware and display too.
In cases of increasing the framerate the experience might even increase.
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As for which quality for your odyssey. (Numbers are approximates, I'm using 110° vFOV => 100° hFOV)
((1440/100)x180)x2 = 5184px
So 5K should be a good fit for you.
Ofcourse, 4800px is also called 5K, so take it with a grain of salt. If your PC can handle it, go for 6K
Also 6K for fisheye. (200°+)