Uni-Kal: Dries faster, hard to work with, a hard surface plaster. Has to be mixed with water. Better if used for small projects/repairs.
Diamond: Takes longer to dry, softer plaster, easier to work with, needs to be mixed with water.
Quality of Uni-Kal vs Diamond product is about the same.
Excalibur: Falls between Uni-Kal and Diamond. The surface is harder than Diamond but softer than Uni-Kal. Also mixes with water.
Joint Compound: Prepared plaster mix. It already has the appropriate consistency to apply without having to mix anything to it. You will need to wait longer for it to dry though, anywhere between 1-3 days to be on the safe side, especially if you will be painting over where you repaired. Joint compound is ideal for small cracks and holes – much less messy, although you may need to lightly sand your finish in a couple of days (which will then cause some white dust), so beware.
Easy Sand 90, 45, 20: Used like joint compound but needs to be mixed with water. The number shown indicates the amount of time (in minutes) for the mix to dry.
Note: You can use an Accelerator to make the plaster dry faster and you can use a Retarder for the opposite route - to slow down the drying of the plaster to give you more time to work with it before it dries like a rock!