Both are decent retellings. However, the French version captures Christie's whimsy better.
Some reviewers will argue that the French versions of Christie's books are more "upfront" about sex. This isn't entirely accurate. The BBC versions have lots and lots of sex. All over the place. Sexual motivations riddle the scripts like candy-corn on Halloween. The scriptwriters double the number of scandals, bringing up "the bedroom" at every opportunity.
I think it is more accurate to say that the French versions are simply less shocked about it all.
As was Christie.
Although Christie provides many types of relationship motivations for murder--adultery, marriage for money, emotional manipulation, abuse, etc. etc.--her overall attitude is quite, well, continental as opposed to English Victorian. Miss Marple sums up Christie's attitude here:
The French or Continental attitude, in other words, sees romance as funny, odd, blase, given to cliches (why not?) but also given to the everyday vagaries of human psychology.Modern novels. Very difficult--all about such unpleasant people, doing such very odd things and not, apparently, even enjoying them. "Sex" as a word had not been much mentioned in Miss Marple's young days, but there had been plenty of it--not talked about so much--but enjoyed far more than nowadays or so it seemed to her. Though usually labelled Sin, she wouldn't help feeling that that was preferable to what is seemed to be nowadays--a kind of Duty.
In Cards on the Table, the BBC falls back on a rather high-toned romance between Ann and Despard, foregoing the far more interesting (French/Continental) Christie plot point that Despard changes his mind.
The opinionated, funny, extroverted Avril (Rhoda) is far more to his taste than demure Ann. This is life. It isn't shocking. It just happens. (And it happens the other way round as well.)
To achieve SHOCK, the BBC substitutes the funny, extroverted Rhoda with the character of an obsessed (probably sexually!) roommate who has been knocking people off to keep her bestie, Ann, with her. Rhoda becomes an unbalanced sociopath while Ann retains all of her demure innocence. Ann specifically loses the speculative self-protective craftiness that makes her such a credible Christie murderess.
It isn't that obsessed roommates don't show up in Christie. It is that they are far more plausibly and cleverly crafted when they do.
Recent BBC versions of Christie often seem plagued with the need to disprove Freud's comment Sometimes a cigar is just a cigar (if he ever said it). No, no, proclaims the BBC. That's never true! What's behind the door is always shocking!
Both the English and the French have bedroom secrets. The French just don't think bedroom secrets are anything to get excited about.
Neither did Christie.