Stiff-  a dense polycarbonate material, the stiff material selection is incredibly bright and responsive through a noticeably stiffer range.  Glassy in feel, clear in appearance, and "tuned" to tightly strung instruments or string picking positions, the stiff material produces the loudest and richest tones available from your acoustic instrument.  This material selection may be tough on the life of your strings.  This material is also a little insistent; if you have a tendency to contact the string with the flat or deeper portion of your pick or use styles that use other than the center of the tip to strum this material may present a bit of a challenge.  The real advantage of this material is speed and depth of tone- the tip and body of the pick in this material work together in amazing ways to store and release energy through the string and recovers from tip deflection in an instant.  This material might be a good choice for you if you "tap" the top half of the string, if you play without bending the string much, if you want full, bright tones from your acoustic instrument, or if you have a good feel for "rolling" the edge on and off the string without much pick penetration to the string bed.
 
 

Flex- a super tough acetal polymer blend, the flex material is a more forgiving selection.  Bright and serene sound, with little apparent pick noise, the flex material allows a deeper string attack with more tip deflection and control by your grip.  The flex material is opaque, with a little spongier texture, and is a strumming, chording, and back picking dream.  The modulus of the material is suitable for more loosely strung electric instruments, although a sound favorite for many acoustic instruments.  The real advantage to this material is edge control and feel; the sounds produced by this material are very woody and natural.  The flex material is durable beyond belief, and allows a more relaxed and controlled grip.  This is your material selection if you prefer a very light, deformable tip strike on your strings or if you pick right in the body of the string.