Metal mags are nice. Hefty, solid, nice-feel/sound when you insert them. The insert (the actual guts) can pull out from the metal shells. Carrying 10+ gets real hefty by the end of a long day.
Plastic mags feel like toys. Cheap in cost, you won't cry if you loose one in the dirt somewhere over the last hill, they may split (just crazy glue shut) but won't pull apart. Sooooo much lighter than metal mags at the end of a long day.
Buy a box of each....sell the ones you don't use and buy more of the ones you do.