what item is better for a steelix to hold iron plate or metal coat and why
They both offer the same benefits (20% increase in steel type attacks) so it doesn't really matter which one you use. I have my steelix hold it's mega-stone anyway, it looks really cool and has one of the best def stats in the game :)
I think the metal coat because if you trade an arceus from your older games (if you have any) you can make it a steel type using the iron plate.