I think I read somewhere that Southdown is no longer being distributed anymore. The only Southdown you'll find is leftover pallettes from last year's stock.
Some people use silica sand with little to no problems. However, silica based sand does increase the production of diatoms in freshwater, I believe it does the same in saltwater as well. Also, many people prefer to use calcium carbonate-based substrates due to the ph buffering and the smaller grain size.