S vs RG- the winner here is the one which is preferred by the player, no 1 model is superior to the other.
the ZR vibrato has a locking mechanism to keep the vibrato in neutral position should there be a change in tension. again, the superiority here depends on the need of the player. if you think this locking contraption has something for you, by all means, invest in one. otherwise, bear in mind that the ZR unit weighs more than the average Edge vibrato.
there are both S/ RG reviews in the database if you are keen 8)