They're both progress for some uses (and users).
HAP-40 is Toshiba's powder-metallurgy version of M4 HSS. M4 contains about 5% Vanadium, and the resulting Vanadium carbides can't be directly cut by anything short of CBN or diamond. Softer sharpening media will simply "erode" the ferrite matrix around those carbides until they fall out. HAP40's grain size is smaller than that of conventionally processed M4, which means that you can get a decent edge on Alumina as in your ceramic stones, but in my experience you can get noticeably better results with diamond (I have a set of HAP40 oire nomi).
I decided a while ago that I don't want to muck around with diamond for routine honing in my shop, so the HAP40 chisels don't see as much use as my PM-V11 and White, and HCS/O1 ones (all of which hone nicely and easily on Alumina).