the fibers are the same, the size is difference. the size of the 187 is slightly bigger than the size of a normal pigment jar, whereas the size of the 188 is slightly smaller than an eyeshadow pan, if that makes sense in mac-language, lol. i prefer the 187 for foundations, and applications of full-face products (MSFs, beauty powders, and bronzers), and i use the 188 for contouring, blushes,and blending. it's not absolutely imperative to have both, because the 187 will do just fine, but it is a convenience. i couldn't live without my 187s, but i just recently picked up the 188. i don't use the 188 as much. i would say the 188 is about half the size of the 187.