I've tried aluminum, steel, alu with steel sleeves, and plastic, and plastic was for sure the best, and aluminum was the worst. But yeah, steel is way better than aluminum.
but my chromo is from like 2011-2010 and its only worn like half a millimeter all round, and my alu's are from like 2011 and worn a bit more so
i reckon alu pegs slide alot better on metal/rails but dont last too long, and chromo pegs grind kinda eh on metal but much better on street ledges and last ages