knowing nothing of these things, but being desperate to use my new Christie's catalogues covering the Parkington Sales, I've just compared the shapes mentioned here against those shown in the catalogues.
Although Michael Parkington appears to have had a lot of Monart, he seems not to have had shape PD, at least I can't see it in the catalogues, so would seem to be rare as Gary says.
Unfortunately, not all of the Monart pieces in the Chrisitie's sale catalogues were provided with a shape code.
On the other hand Roberta's HF shape seems to be less scarce, although if the Parkington papers are anything to go by then uniform mottled colours like her piece are not so common - whorls and aventurine decoration look to my inexperienced eye to be more the order of the day - at least for the HF shape. But it's a nice bright piece, and I'd love to find it.
Must admit I never see Monart pieces, perhaps I'm just not looking for them - but then again I daren't start another area of collecting - some of these things do look to have large footprints, and I'm in trouble with space as it is
Anyway, both are great pieces, but if pushed I'd have to choose Roberta's simply for the more attractive colour