I did both editions simultaneously during my recently-ended 6 week sprint and thought that my feedback would be helpful to those who are considering using LP for French. Before I begin, let me stress that I was not learning French for the first time, but was rebooting it after an eleven year, post-IGCSE hiatus.
I would view both editions as being excellent, with a preference for the 1950 edition because of the following:
1. The 1950 version has short, easy-to-follow lessons that do not strain one's attention. The notes are excellent at explaining the grammar (yes, I am a grammar pedant ), vocabulary and the cultural context in which colloquialisms are used. (In comparison, the 1971 ed has 30 long lessons divided into 3 sections.
2. When LP created the 1950 version, it made all language courses in the same era the same. Ie, lesson 23 in French would be the same in Spanish, German, etc. This makes learning subsequent languages an absolute delight!
3. The LP 1950 course felt more comprehensive than the 1971 one, despite the former having taken me a shorter time to complete than the latter. LP 1950 is better for someone doing a language challenge than LP 1971.
One gripe that I do have with LP is the number of books that the courses come with. I would rather have two voluminous books, than four smaller books any day. I would have preferred it if they had adopted the Assimil method and had translations and grammar notes on an adjacent page (and not in other books!).
Using Assimil FWOT as the benchmark (ie, 10/10), I would give LP 1950 9/10 and LP 1971 8/10. (For reference's sake, Living Language Ultimate: Beginner-Intermediate gets 7/10 and Assimil NFWE gets 8.5/10.)
Would I use LP for learning subsequent languages? ABSOLUTELY! I have already bought editions for ten other languages!