the difference between the two operations (turn off trace reflectivity versus turning down the reflectivity to zero) is that the first one deals with the object ONLY... you can have several objects with the same material and ONLY that object will not reflect - whereas with the second option, all objects with that material applied won't reflect.

and as you can see in mayaniac's second image, it'll still catch specular highlights since that's a different value that gets altered in the material.