For quick acting valves, to absorb the inertia, it is better to have the arrestor near the valve. An expansion tank could act as one if the water pipes coming in were banging, but wouldn't help the water column still moving after it nearly as effectively as one at the end of the run near the valve. NOw, if there was a PRV in the system, that little extra water to push the arrestor would store a very small amount of water under pressure, but it would be release as soon as another valve was opened in the house.