In this case, they both have major merits. The Schlesinger pulls out all the stops visually and atmospherically, through the many recognisable Dorset locations - see the IMDb list - and its wistful soundtrack integrating thematically pertinent English folk, (such as The Bold Grenadier) and minor-key English pastoral score by Richard Rodney Bennett (also see YouTube - ignore the idiosyncratic animation). On the downside, I think Julie Christie as Bathsheba Everdene was miscast and the role misinterpreted, making her a smug tease rather than inexperienced; and the plot, though excellently adapted by Frederic Raphael to maintain the flavour of the novel and include some of the more distinctive dialogue, excised a deal of storyline in favour of clear-cut interpretation. For instance, the film shows Gabriel Oak wading in on minimal acquaintance to ask Bathesheba to marry him, making her refusal unsurprising; but in the book they actually have much more of a history (Bathsheba saved his life from smoke suffocation after he fell asleep in a badly-ventilated hut). Another omission was the fate of Boldwood, whose death sentence is commuted on grounds of insanity, following a petition, after the extent of his obsession is revealed. The 1998 version (if you don't mind Japanese subtitles, there are clips on YouTube - search for "FFTMC") is an altogether more low-key treatment, but incorporates much more of the text and considerably more subtle characterisation. For example, it follows the book in showing Bathsheba maintaining her composure after Troy is shot
"Gabriel." she said, automatically, when he entered, turning up a face of which only the well known lines remained to tell him it was hers, all else in the picture having faded quite. "Ride to Casterbridge instantly for a surgeon. It is, I believe, useless, but go. Mr.Boldwood has shot my husband."
- Far from the Madding Crowd, Gutenberg EText-No. 27
as opposed to her instant hysteria in the Schlesinger, notable for its 30+ consecutive utterances of "Frank" (this must be a record). I think the Renton wins on intelligence and authenticity to text; Schlesinger on emotional punch in conveying the "partly real, partly dream-country" nature of Hardy's Wessex.
That said, it's open to discussion whether authenticity to text is an automatic merit; an adaptation is a derivative work, and can do as the adapter likes, which may be an improvement. (In modern context, I can think of the Inspector Morse TV series, whose depressed intellectual hero is far more iconic than the rather seedy Morse of at least the early Colin Dexter novels; and the culturally-allusive and Oscar-winning Shrek animations compared to William Steig's naive and cumbersome picturebook).
Which text anyway? Far from the Madding Crowd first appeared in 1874 as a anonymous Cornhhill Magazine serial illustrated by Helen Paterson Allingham (see the Thomas Hardy Association site, or The Victorian Web for a detailed commentary); Hardy made major revisions for the 1895 book edition, and more in 1901. The term "Wessex", for instance, made its first brief appearance in the 1874, but Hardy later expanded references as it grew as a brand - see Hardy perennials).
While skimming, I noticed one such Wessex reference that I haven't previously seen explainaed, at the opening to Chapter 50:
THE SHEEP FAIR -- TROY TOUCHES HIS WIFE'S HAND
GREENHILL was the Nijni Novgorod of South Wessex; and the busiest, merriest, noisiest day of the whole statute number was the day of the sheep fair.
This refers to Nizhny Novgorod, trade centre of the Russian Empire, which similarly had the annual Makaryev Fair. As the paper Hardy's "Pedantry" mentions (CH Salter, Nineteenth-Century Fiction, Vol. 28, No. 2 (Sep., 1973), pp. 145-164) the use of such erudite allusions got Hardy a deal of stick from critics. William Sharp (Thomas Hardy and his novels, 1892) called him "in point of diction the most Latinical writer we have had since Dryden and Milton". Argument about Latinisms is nothing new, then.
I'd quite forgotten I have a copy of Thomas Hardy: The Critical Heritage (RG Cox, Routledge, 1970) which collects contemporary reviews of Hardy's works. Much of the 1995 edition is viewable at Google Books, and the Far from the Madding Crowd reviews make interesting reading. Although the novel more or less cemented Hardy's reputation, the general tone of reviewers was to praise the originality of setting and scenario, but criticise the style. The unsigned Athenaeum review noted the "penny-a-liner" phraseology and Hardy's habit of putting astonishingly erudite language into the mouths of supposedly illiterate countrypersons
"Yes." continued William," they pranced down the street playing "The Girl I Left Behind Me." so 'tis said, in glorious notes of triumph. Every looker-on's inside shook with the blows of the great drum to his deepest vitals, and there was not a dry eye throughout the town among the public-house people and the nameless women!"
RH Hutton, in The Spectator, echoed this view that Hardy's rural characters are uniformly too erudite for belief:
The whole class of hoers, sowers, ploughmen, reapers &c., are – if Mr Hardy’s pictures are to be trusted – the most incredibly amusing and humorous persons you ever came across, full of the quaintest irony and the most comical speculative intelligence.
Andrew Lang, in Academy, spotted this too, further noting that the country folk in the story had "not heard of strikes, or of Mr. Arch". This comment deserves more analysis, in that "Weatherbury" is based on the real-world Puddletown, only a couple of miles from Tolpuddle, home of the Tolpuddle Martyrs. Despite the authenticity of detail in many respects, the Far from the Madding Crowd novel doesn't appear to be socially and politically savvy. While Hardy's Wessex is anything but idyllic - personal tragedies abound - it does embody a very recognisable stereotype of the timeless countryside to which Hardy adaptations, particularly the 1967 film, do tend to play. As I've said before - see Views of the countryside - it's a very English yearning, and I think Hardy played a strong role in fostering it. Wessex as "partly real, partly dream-country" goes deep.