Oracal recommends 651, 751C and 951 on their website:
http://www.orafol.com/gp/americas/en/products/plotter-films/marine-graphics-9765
The major differences are Life Expectancy and Shrinkage:
651 - 6 years - colors/transparent/black and white
Adhered to steel, no shrinkage in cross direction; In length, <.016”
751 - 8 years - black and white / 7 years - colors/transparent
Adhered to steel, no shrinkage in cross direction; In length, <.006”
951 - 12 years - black and white/ 10 years - colors/transparent
Adhered to steel, no shrinkage in cross direction; In length, <.004”
Between 751 and 951 there seems to be a 30% price difference for about a 30% increase in life expectancy.