Not sure which product of the two companies you are referring to.
However the Truck Bed Liner that Rustoleum sells is great stuff in my opinion. I have used it on everything. Holds up real well. Even stands up to some hard tree rubs. You can wash mud right off it. I pressure wash everything on my Jeep and it stands that real well.
I have used it in places on my rear fenders for a blast shield as my front tires just peppered the leading edge of my rear fenders. Stands up well in that application too.
Not hard to apply, just shake the snot out of the can. Do it when its warm out. I even use a heat gun set to 200 to warm the part for a while after application. Kind of a hillbilly baked on finish.
It will hold up real well on rock rails. Oh and easy to refinish if need be.