The premixed stuff is not plaster at all.
It hardens strictly be evaporation of the water.
The setting type compounds ARE a plaster product and harden by chemical reaction.
They are far supperior to the pre-mix buckets, and if you want a surface as hard as lime-plaster Durabond is the ticket.
It is nearly impossable to even sand, just like real plaster.
Easysand is more forgiving.

I recall reading that you can mix two together , 20 & 45 for example, and get a setting time somewhere in between. Anybody know if that's really the case?

Maybe I'll try it when I have some of both left over.

On a separate note, what's the real deal with just using sheetrock "All purpose" compound for all coats? Our county government is doing a major interior makeover, which is still open to the public, and they were using "All purpose" for all coats. As for their technique and workmanship, they're real pro's. These guys were doing an atrium balcony wall's second coat (4 ft high wall), across the entire 4' height with a short piece of steel stud. I was amazed it was coming out so even (two guys working as a team). It still looks perfect 6 months later.

Do not mix premix mud with a setting type compound, as they are two different animals. If you are not skilled at floating, and will require much sanding, stay with the pre mix. Even though the newer light weight setting compunds are sandable, they do not sand easily like the all purpose. By all means avoid the older "Durabond" setting compounds as they dry rock hard and will not sand at all.