The AR will allow you to use a thinner material for equal strength and provide better wear resistance ( AR stands for abrasive resistant). It is much harder to bend then A36 at equal thicknes, this allows the grapple to be lighter for a given strength.
A36 is pretty much used by companies trying to save money as it's a little cheaper than AR plate.
To give you an idea of the difference:
A36 has a yield point of 36,000psi and a tensile strength of 58-80,000psi
AR400 (I don't have the 450 specs on front of me) has a yield point of 145,000psi and a tensile strength of 177,000psi. The 450 will be just slightly higher.
The number for AR plate refers to its average harness on the Brinell scale, ex. AR400 can have a Brinell hardness of 360-440.