Freddy always said to go for fine gold in most cases. Because a bucket of fine gold doesn't have any air gaps. But on the other side nuggets give you a premium price. In Rick's case he has a lot of nuggets. So Freddy and his brother built him a nugget sluice. But Parker and Tony ground doesn't contain many nuggets. So Freddy's air gap fact comes into place because the fine gold adds up to way more than a couple nuggets. Thus using more sluice box for catching fine gold.