Polychromos pencils are oil based, whereas Prismacolor pencils are wax based - so the coverage will be a bit different - you'll also notice that the oil pencils have less breakage issues and the leads tend to stay sharp longer. It comes down to your own preference... they are different kinds
of pencils, so it's hard to say which is better - each has its place. Know what I mean?
As far as the Prismacolor pencils produced in Mexico... if you dig back through earlier pages in this post you'll find some discussion on this topic... Prismacolor says there has been no formula change, and they stand behind their product, and will happily replace any pencils or product you find to be unacceptable. I have not had any problem with newer pencils not blending. I didn't see any difference in the pencils except maybe a very slight difference in diameter.