Love in a Cold Climate is certainly the funnier of the two books; and it would have worked really well as an ironic counterpoint to the more tragic plot of The Pursuit of Love except that I felt Megan Dodds as Polly really wasn't up to it, and since she is such a central figure that kills the story despite the excellent performances of the male leads in that plot strand; you rather wonder what Boy saw in her. Also, I have to say that my memory of the end of the book was that there is a strong implication that Boy and Cedric have settled down with Polly's mother as a threesome, an aspect which I missed from the TV play.
The Pursuit of Love is probably the better of the two books, but it also wasn't terribly well served by the production; Rosamund Pike had to more or less support Fanny's story single-handed, matched up against a series of unimpressive males (apart from her father). Having said that, the TV format perhaps shows Linda's love-life a bit more vividly than Mitford's original text, and some of the best scenes are when she is a) being converted to communism and b) being confronted with the Spanish civil war; and at the very end, I thought it came out just about right, with Frances Barber as the Bolter coming back into the story and providing an ironic perspective on it all, which I have to admit brought a certain moisture to my eyes as the final titles rolled.
I really wish the 1980 version, starring Judy Dench as Linda's mother and Anthony Stewart Head as Linda's first husband, was commercially available, but you can't have everything.