I've seen a few questions regarding the best kind of polymer clay to use for jewelry, so I thought I would offer up some information.
Clay brands and formulations have changed every so often over the years, and it's likely that they will continue to do so once in a while. However, every manufacturer has a specific type of clay artist in mind so that's who they continue to try to serve. The characteristics of clay for making jewelry are different than those for purposes.
If you are making jewelry, you want a clay that is strong but not brittle. You want a clay that is hard so stringing wire won't cut into it and create gashes around the holes in your beads. You want a clay that is soft enough with which to work, yet firm enough to cut and slice without a lot of distortion.
Of all the clays currently on the market, there are three that best meet the needs for making jewelry. These are Premo, Kato, and Cernit. These three clays have the properties we seek. I have personally focused on Premo and Kato. Cernit has been reformulated most recently and a couple of top clayers have started to use it more for jewelry.
While I use Premo and Kato almost interchangeably, there are a couple of differences which sometimes matter. For example, when I'm doing my highly detailed mini-floral sculpting work I prefer Premo because it's just a bit softer, stickier, and more pliable so my teeny bits and pieces seem to stick better while I'm working on a piece. I use either Premo or Kato for caning, but Kato is definitely the firmer of the two and makes it easier to slice canes without a lot of distortion. (A cane, by the way, is a long tube of clay with a design that goes all the way through from end to end. Every slice that is cut from the cane has exactly the same picture or pattern).
Both brands of clay offer metallics as well, in Pearl, Silver, Gold and Copper. These clays are impregnated with fine particles of mica. When the clay is rolled through the pasta machine into sheets, the little bits of mica all line up on the same plane, creating a surface shimmer effect that clayers use for several special techniques.
The Kato clays are specially formulated to be TRUE ARTIST colors. That means they use the same basic colors that painters use, in true tones. You, as a clayer, then use a color chart to custom mix your own shades and tones from there as desired. Or use the clay as primaries and true-tones as the mood strikes.
Premo clays also come in a full range of colors but they are not necessarily true artist colors. That's never stopped me from using them though, as I rarely use anything directly from the package without altering the color in some way.
Every clay manufacturer, on their web site, has formulas to help you custom mix colors according to the color wheel. There are other web sites as well, where favorite formulas are posted. And most artists who publish books also include some color mixing charts.
Both of these brands also offer translucent clays. In the raw state, they look white. Baked plain (not mixed with anything), they look sort of gray and ugly. Translucent clays are used in special mixing techniques, to create depth and to mix with opaque clays to create soft pastel colors that have a translucent quality to them. Translucent clay frequently is the key ingredient in creating "Faux" jade or opal, for example, by mixing a translucent base with other colors and materials to get just the right look.
One of the other differences between the brands of clays is their baking temperatures. The purpose of baking the clay is to use heat to cause a chemical reaction which permanently binds the ingredients of the clay and causes it to harden. The clay must reach a certain temperature in order for this chemical reaction to occur. That's why you must ALWAYS follow the package instructions for time and temperature. It also makes the use of a free-standing oven thermometer mandatory, as the built-in ones are not necessarily accurate. If the clay is underbaked it can feel hard and seem fine, but it will not be as strong as it is intended to be. And if it's overbaked (burned), it turns dark and makes a big unpleasant stinko.
So, all that being said, do I ever mix brands? Yep. Very carefully, but yes I do. For example, I love some of the colors of "non-jewelry" brands of clay. So I sometimes use just a pinch to tint some Kato or Premo (translucent or pearl). I never use less than 80% Kato or Premo, and follow the baking instructions for Kato or Premo depending on what I'm using.
So, which brand should you use? The response is always to get a bit of each and play around with them to see how they feel to you. Kato has an odor that bothers some but not others. If you have "hot hands" you might prefer one over the other. If you are sculpting, caning, molding, painting, etc. - you will find that your clay preference might vary depending on how you are using the clay.