Well, as you can see, the Bomix is much whiter than southdown.  Bomix has almost a bleached look to it, whereas southdown has a slightly tan colouration.  The grain size is also much smaller with SouthDown as opposed to Bomix.  Both perform very well in the vinegar test and should act well as PH buffers when used as substrate in a Marine Aquarium.

Bomix can be ordered from the Bomix Company and delivered to your local Rona/Cashway store.  The exact link to the Bomix White Playsand on Bomix's website is here.