Good question...I wasn't aware of that!
I believe I have the DH + IH one. I'll try to remember to check when I get home tonight.
I'm pretty sure that one is effectively an update of the PB version, both being WRG publications after all, albeit continued and expanded by different authors. So differences will reflect changes in opinion or more information having come to light over the previous decade. And therefore the later one can be assumed to be better. Probably
Of course, thinking may have changed again since 1982 (was it really that long ago? Blimey, how time flies.). Almost certainly in fact! So there may well be other better reference works available nowadays.