When deciding on a favorite brand, be sure and notice several things:
...brands have changed formulas (many of them quite recently) and sometimes that means the stuff on the shelves they will be softer than it would have been in the recent past or than bars you'd bought previously ...the newer versions may also have been manufactured much more recently so those bars probably won't have been allowed to "advance" naturally with age at the manufacturers like they will in the future
...single colors within a brand can be different hardnesses naturally (because of the pigments used to color them, or because of the type of plasticizer available for purchase the day they were manufactured), or later as a result of too much heat or UV light at a supply house, in delivery trucks, or on shelves at stores (...stock currently available at different stores will vary too)
The softer clays may be easier to condition, but they sometimes give up strength after baking (in thinner areas) for that softness, as well as giving up good handling and the degree of detail they can achieve. A good example is Sculpey III... it's very soft when raw, but gets mooshy pretty quickly with handling (especially in warm situations or for those with warm hands) and it's also is easily broken after curing unless the clay is very thick (the stronger clays will be Premo, Kato, FimoClassic, and even the newest version of FimoSoft).
Since clayers often use aids to condition their clay (pasta machines, softeners, etc.) and it's not a big hassle, they often prefer the firmer brands because once those are conditioned, they're really
wonderful to handle --plus, items will often turn out better than if they'd been made with a softer clay.
Otoh, if a bar has no
give at all when squeezed at the store, it's usually not worth the hassle of conditioning it.
As for storing any polymer clay, the clay will firm back up after the heat has dissipated from being handled (which is one reason why it can be good to let the clay rest overnight, or even a while, before trying to cut slices from canes or get more detail).
Over weeks and months, it will firm up a little more (but not as much as before it was conditioned). And though it will always be usable with more conditioning, it can be kept as soft as possible in an air-tight situation ...for example, wrapped in plastic wrap or a baggie (most
brands are okay for clay contact), then placed in a lidded container away from excess heat and UV light.
If you're interested in reading more about storing polymer clay, check out this page for lots of info:http://www.glassattic.com/polymer/storage.htm
(and if you're interested in ways to condition clay, check this one:http://www.glassattic.com/polymer/Conditioning.htm
...and this page if you're interested in the "characteristics" of the different brands:http://www.glassattic.com/polymer/Characteristics.htm
(All the brands and lines of polymer clay can be mixed together as well...the new clay will have characteristics proportional to the amount of the clays that were used.)