Like a lot of others, I did my own 'test' of different clays used in moulds. I mostly have Prima moulds and really want to use them, but found that some clays are difficult to work with and I've become discouraged. I'm working on a project that will have 6 alike moulds on it. My internet research led me in a great direction to find what I consider to be the best clay ever!!
This is the result I came up with. I hope it helps someone, but please remember that these are my own personal opinions. Since the writing would be way too long, you can read the detailed comparisons of these three clays on my blog if you're interested. NOTE: I believe that Prima's re-design Paper Clay has been discontinued. MY TOP CHOICE is "Polyform Model Air Clay". It is the least expensive of the 3 that I tried. Bought 2.2 lb package at Michael's and paid less than $6 incl tax using a 50% off coupon. My blog has comparison details (photos and written) if interested -