the southdown,is fine grained adn will be ok for the tank, thesmaller grain size is actually better for the tank then larger grain, diversity is good too, but the finer offers more surface area thus making it even more effective for bcteria colonization and nitrate reduction
also, silicates have been rumored to be a problem for algae, but if nothing else is available to you, then silca based sand is a better otpion than none at all, so if you cannot get sd arag, then the silica sand will be ok, but just not quite as good
HTH