The Commodore is the "standard" wheelbase family car in Oz, and is what the SS is supposed to be. Highway patrol are mostly SS Commodores and XR-8 Ford Falcons.
Here, the Caprice is part of the Statesman range, which has extended wheelbase for more rear legroom. Caprice has the higher luxury level, and the Caddy style wreath emblem.
(Got me a 1997 Caprice, L67, leather lounge to drive to work in...bloke paid $67k in 1997, I got it for $9,300 in 2012, with less than 63,000 miles).
If they (Holden) sold an optioned down Statesman in Oz (LWB, more rear room, and no need for leather and such), they would sell heaps of them