this is what it all comes down to:
if you buy ca gp ics armalites you are willing to settle for whatever metal body and internal parts they give you.
if you buy a marui, you put whatever parts you want on/in the gun without compromises.
metal breaks too. just because its metal doesnt automatically mean its stronger. it only makes you think the gun is stronger. some of the metal used in airsoft is actually very brittle so while it may be less prone to warping and wiggling it may shatter on impact.
take a look at this metal bodied m4 after a fall:
this is only the upper you should see pics of the lower, its in even worse shape.