If you are talking comparison, you need to ask a single simple question. I will use a sight as an example. What is the advantage of a particular part being plastic? Price should be the primary advantage of a plastic product. That's it..... If the price is the same as a metal alternative design, well, you've just realize who's making a larger profit margin!

Plastic is lighter than aluminum, but most designs that use plastic to replace a metal part are reinforced with additional material to meet the strength requirements. Again, in most applications, plastics are used to reduce cost - period!