When a spinning reel comes with 2 spools.,Which is better to use and why. Aluminum or plastic? What benefit does each different spool provide Or is it just a matter of personal preference?

First off it isn't plastic it is in fact graphite. You paid for the aluminum spool and they give the graphite free. As for benefits none that i'm aware of, which one you use the most is personal preference, the second is usually backup.

Tight Lines!!!

If it comes with a plastic one throw it away(plastic spool that is)...reason why?Because plastic ones have 2 molds which are welded together...if you knotice where they join together they have sharp edge that'll cause your line to rub and weaken your line.Everytime you run your line over those you are knicking your line.Now i have heard people sand those down a little...not sure if it helps unless you can get that incredibly smooth.even then,you are roughing up the surface by sanding it down.

It's better to get the aluminum ones..they are one piece and much,much nice.Might be a bit more costly for an extra aluminum spool,but worth it.Alot of spinning reels comes with extra plastic ones for free....if that was me i'd stay away from the plastic ones to be honest.

"Cheap" reels come with free spare graphite spool ( < $50 ) slightly more costly reels ( $60+ ) come with free spare aluminum spool.