Eames molded plastic from Herman Miller or the Fiberglass shell chair of Modernica?  

This has been discussed at...
This has been discussed at great length here; you can find the earlier discussions using Google ( http://www.google.com):
http://www.google.com/search?q=site%3Awww.designaddict.com%2Fdesign_addi...

thanks for the hint!
thanks for the hint!

Plastic
In my opinion the plastic is a natural progression from the original 1940s experiments with Eero Saarinen in plywood, to the stamped aluminum chairs - which were not produced because the material wasnt quite right, to the pressed fiberglass. Whenever a new, better material presented itself the Eames used that.

You can see
You can see the same progression of materials in the DCM - which originally existed in plywood, with shockmounts to the EA127, essentially the same chair as the DCM, except with an upholstered molded poly shell with incorporated built in mounts (in the same way the re-issue shells do).
And you can see it in the upholstering methods of the plastic shells: starting with the loose cover wired on, to the cover hand stitched on, to the cover sewn on and foam vacuum molded to the shell.