For Superstar the original concept album is the one I grew up on (it was an insanely popular album when it was released, even amongst those who were not typically theatre fans). My dad had the original 2 LP set and I put those onto cassette for myself. Thus that's the way the show has always sounded to me, and I find a lot of later recordings, especially from the 90s onwards, tend to have performers who sound too clean and polished to my ears and lack the rough and raw power of that original recording.
I do currently have it set up as a playlist to include "Could We Start Again, Please" from the movie soundtrack. It still flows just fine, since it is Yvonne Elliman playing Mary all the way through.
I will say I don't really care for the Original London Cast recording of Sunset Boulevard. The levels, especially between the scenes and the songs, are all over the place. If you want to hear half of what Patti is whispering in the dialogue tracks then you have to crank the volume up to high, but be ready to slam it right back down when she starts belting. It doesn't make for the most relaxing listening experience.