i have something like the 60th anniversary edition. i presume it's the 50th because i've had it a long while. it came with 'unfinished tales' in the same collection, too.
i would presume that the 'anniversary editions' are banking on (a) being a limited print run, and (b) becoming collectible. it's put out by his main publishers, harpercollins, so will be a good faithful reproduction/restoration of the text. can't go wrong there. i'm not sure it's worth the cash, personally, but that's what these 'special editions' are all about.
the other thing is that harpercollins, being the official publisher, will have had christopher tolkien onboard. he was a known meddler and exerted very tight control over the estate. thus you get this:
n 2005 Tolkien's text was fully restored - with the full co-operation of Christopher Tolkien - with almost 400 corrections, the original red and black maps as fold-out sheets, a fully revised and enlarged index, and for the first time a special plate section containing the pages from the Book of Mazarbul, making this set as close as possible to the version that J.R.R. Tolkien intended.
only they can say that, because they had christopher's blessings ($$$).
the folio society are known for agreeing deals with publishers and re-packaging classics in very nice presentations. we're not talking leather-bound don's study, of course, but they make 'nice things' for general readers. deluxe paper, very nice typesetting, very nice detailing. it will be a higher quality product than the harpercollins one, where the high cost is in it being 'definitive'. i can see the folio edition includes a frontispiece with many illustrations/etchings, including by eric fraser, who was a very noteworthy illustrator. it'll be a much nicer version for presentation's sake.
Eric Fraser was one of the foremost British illustrators of the 20th century. His images – a total of seven full-page images and 57 head-pieces – are based on original designs by Queen Margrethe II of Denmark. Tolkien had seen her sketches and was so impressed that his executors gave special permission for Fraser to interpret them for these Folio editions. Covers blocked in gold and maps of Middle Earth as endpapers complete this superb edition of one the world’s favourite stories.
so both editions are 'canon', if you will, i.e. both approved by tolkien or christopher tolkien.