Despecialized are sub-720p edits based mostly on the Blu Ray 2011 release of the original cuts, with some parts sourced from LaserDisc, DVD, film reels and original matte paintings. It’s a work of love that took the HD Blu Ray version and video-edited frame by frame it to look like the original release. It’s a much better watch than the special editions rips, even though those rips have better resolution.
The 4k editions are high resolution scans from original film reels, they are typically found as 1080p and 4K.
If you don’t mind 25-50 GB per movie then the 4k are the way to go, otherwise get the despecialized (they’re under 2GB).