The total number of bitcoins that can be mined has been preset at 21 million, whereas the amount of gold that can be mined is a more complicated question. Over time, the increase in the supply of gold has increased at roughly 1-3% per year, but on a short-term basis there can conceivably be a lot of volatility in the price of gold due to new discoveries. Of course, as a new technology, bitcoin is subject to much more volatility than gold. But over the long run, given the fact that no new bitcoins will be mined after the 21-millionth, we can expect it to ultimately serve as a better store of value than gold.