Modroc-very simple and easy to use, modroc will work over pretty much any material, it doesn't dry up too quickly and it moulds decent to the shapes you give it (Although it doesn't always pick up the smallest of details), it's also activated by water. I would use modroc quite a lot if i had the right project- in my opinion it's good to get a base shape and it really easy to work with, here are some examples that are made out of modroc.
Next i looked into plaster which is used much more often, it gives a more professional type feel, although i think there's a lot more to it. Plaster is typically used for casting. It starts off as a dry powder, when mixed with water it heats up and thickens. Plaster fits very tightly to shape and normally picks up details very well, the downside is....it drys....quick!
Artists tend to use plaster to do casts of themselves and others, here is an example.
Here are some examples to show that plaster isn't only used for casts.
Plaster seems very versatile, and hopefully next time we use it, i'll feel a little better and will be up for it, because i'd really like to give it a try.