It turns out I read these two out of sequence, as I had thought A Murder of Quality came first, but it doesn't really matter. I was prompted to go back and re-read them after watching Mark Lawson interviewing le Carré a while back on BBC4, in which he said he was "embarrassed" by these early novels. I think that's harsh, though I can see perhaps one or two ways in which they do have a less developed style.
Call for the Dead begins in perhaps a rather stereotypical way, with an apparent suicide of a civil servant who had been under investigation by a certain Mr George Smiley. Interestingly, Guillam and Mendel make their appearances straight away, but I don't think there are any other characters to recur in later works (except distant references to already long gone people like Jebedee). It's full of clever twists right to the end, and if anything is out of later character besides the opening, it is perhaps that Smiley has a bit more of a Harry Palmer character in this one.
A Murder of Quality, on the other hand, moves away from espionage altogether, with only a thread of wartime colleagues to link Smiley to the other characters. This one is more like Christie in many ways, and perhaps feels less original; the ending is rather sad and sordid, in a way that is of its time.