The Crayola is softer and much more expensive.
We can buy Kleen Klay here in Australia for about $7 a kilo.
Kleen klay also comes in three hardnesses: Extra Hard, Hard, Meduim and Soft. I usually go the hard Klay as it seems to resist my sometimes rough handling of part-finished moulds (I am a clumsy bugger) and gives a cleaner line to two part moulds IMO.
I warm Kleen Klay up in the microwave, about 30 seconds for each 500 grams does the trick without it getting too hot, prior to using it.
I have also used a clay product (below) that is designed to be melted and poured into moulds for stop-motion production. I have found it to be pretty good to make two part moulds with pretty clean and level parting lines, by simply pouring the melted clay into a formwork up to the desired level of the parting line.
http://www.barnes.com.au/catalog/produc ... cts_id=775Care must be taken not to get it too hot, as it can bubble a little when poured into the formwork and then the surface needs to be cleaned-up to make a smooth surface.