I use both (on the same rifles, even). Each has its own positives and negatives. No way would I put a vertical grip or sling mount on a plastic rail section. I tried that on my MOE handguards, and I can warp the rail by pulling down on the grip. It may never break, but I don't like that. I usually use them for my flashlight mounting rails, because if something hits my light hard enough to break the plastic rail, I would hope it would break instead of my more expensive light mount.
- Are great for flashlights or anything where you don't plan to yank on whatever's attached to it
- Are cheaper
- Are lighter in weight
- Don't get scratched up and go shiny on you like aluminum
- Are stronger and more trustworthy when attaching a sling point or vertical grip
- Cost more
- Are slightly heavier (no biggie on short rails)
- Will scratch up and be shiny aluminum
There are probably more pros and cons for each, but that's the only gripes I've had with each type.