There is also an online store - Brick in the Yard (BITY) that sells Klean Klay at a decent price and reasonable shipping cost - owner's name is Mitch.
I personally use Klean Klay firm. Primarily because I live is San Diego and the temperature is warm enough to keep the clay pliable. Also, I go with the firm is the forgiveness. When you break for the evening, the next day your sculpt will have stiffened a bit from the clay cooling. With the sculpt a bit firm, I am less likely to "nic" the clay and screw up my prior work.
Another trick to soften your clay - I put my clay in a disposable aluminum pan (like you bake a turkey), then I put an electric heating pad under the tray. Put a towel over the top of the pan - wait 15 minutes you have a pan full of soft clay.
Just my 2 cents.
Elder - One