RickW: ...as pvc only requires half the fall as steel. In terms of personal experience I’d recommend steel every time assuming you can get the correct fall.... Rick
I think that "half the fall" promoted by the plastic boosters is quite misleading. It relies on the low friction of pristine new plastic and does not take account of the increased friction caused by the deterioration of the plastic surface with gravel/sand abrasion in the gutter, nor the saggy/baggy effect of the great thermal expansion/contraction of PVC cf metal.
Dunno what the guide is for steel, but Marley prescribes 5mm fall per 10m which is somewhere between bugger all and impossible to measure!