.999 is the only way to go. I do think in how you are referring it, it is 24-karat gold. (Correct me if I am wrong). When investing, the easiest way to calculate the actual value of the gold you have, is having in it's purest form. I know pure gold is a bit soft, so that .001 doesn't bother me too much. Although once it got to .003 or more missing, I would start to be wary.
One Karat means that the object is 1/24 gold or is roughly 4% gold, though it’s hard to find one karat gold jewelry. 10-karat jewelry, which is very common among less expensive pieces is about 42% gold. 14K and 18K pieces are about 58% and 75% gold respectively. 24 karat gold is approximately 99% or higher in gold content.
I have never bought gold other than 24K. 22K might be good for jewelry... But I don't think it is suitable for bullion.
