The OBC is without a doubt the best cast recording of this show. The current CD edition includes extra material left off the Lp and first CD releases.
The RCA disc of teh 1975 London production is vastly inferior, and at this point I belive it is out-of-print.
The 1977 film soundtrack has never been released on CD and teh Lp reveals a score decimated with more than half of the songs removed, and some odd lyric changes but it has the rewritten "Glamorous Life" (also a bonus track on the OBC Cd release) and some interesting lyrics to the opening "Night Waltz."
There is a studio cast CD on TER that John Yap recorded in the early 1990s. It's worth a listen even if it can't touch the original.
The 1996 London revival as noted is hard to find though apparently in the U.K. there are budget releases that don't actually label it A LITTLE NIGHT MUSIC. Instead it is called something like Judi Dench sings Sondheim! The original contracts restricted the CD from being released (legally) outside the U.K. and that is why no one has ever picked it up for U.S. distribution. I would list this as a second choice after the original but since it is hard to get, why not stick with the original which is readily available and at a budget price now.
Cast albums are NOT "soundtracks."
Live theatre does not use a "soundtrack." If it did, it wouldn't be live theatre!
I host a weekly one-hour radio program featuring cast album selections as well as songs by cabaret, jazz and theatre artists. The program, FRONT ROW CENTRE is heard Sundays 9 to 10 am and also Saturdays from 8 to 9 am (eastern times) on www.proudfm.com