Just posting to cast my vote FOR southdown.
My first tank was a 55, 150- pounds of southdown sand, 20 pounds of actuall live3 sand top seed it-- worked great.
I also set up a 2nd 55 in the basement- it is my 'refugium to be' with another 150 lbs of southdown and 200 pounds of live rock.
i have never had much of a problem, other than a small amount of cloudiness for a couple days.
I also have a mess of bags of these in the garage-- if anyone is in the Buffalo, NY area and needs some, let me know. I need to save some for my seahorse tank and my 'live sand project' but i suppose i could let a few bags go for the right price....
Joe