Even as the stock market sets new all-time record highs, gold and other precious metals are having a tough time keeping up. But as prices fall, the big question that could determine the future of the long bull market in gold, silver, and other precious metals is this: Will investors flee from the metals ETFs that have created so much of the excitement in precious metals in recent years?

From seedlings to market-movers
The growth in metals ETFs has been truly spectacular. In less than a decade, exchange-traded funds have gone from small players in their respective markets to become some of the most popular ways for both institutional and individual investors to make calls on precious metals.

The biggest metals ETF of them all, SPDR Gold (GLD 0.56%), is arguably the best example of the pace of growth in the industry. In late 2004, the ETF came out and quickly gathered 95 tons of gold bullion worth about $1.3 billion by the end of the year. From there, though, demand took off, with the ETF hitting 260 tons by the end of 2005 and 450 tons the following year. Even the stock market meltdown didn't slow interest in the fund, which hit the 1,000-ton mark in February 2009.

But lately, SPDR Gold has experienced something it hasn't seen much of before: outflows. The fund has seen a drop of 105 tons so far this year, and combined with weak prices, that has shrunk the ETF by about $9 billion.

Are other metals ETFs following suit?
Before concluding that metals ETFs are going out of style, you have to look beyond gold. Especially when it comes to precious metals, money often shifts from metal to metal as trends change.

Looking at silver bears out the substitution theory. iShares Silver (SLV 0.82%) enjoyed a similar growth curve to SPDR Gold, growing from around 2,500 tons shortly after its 2006 inception to more than 10,600 tons today. Yet for silver, the big outflows came when the metal's price dropped from highs near $50 per ounce in early 2011 to around $30. Since then, investor interest has been relatively stable, and year-to-date, the ETF has picked up 500 tons of silver, more than offsetting the price declines since the beginning of 2013.

Popular platinum-group metals ETFs tell a similar story. ETFS Physical Platinum (PPLT -0.08%) got popular very quickly after its offering in early 2010, and it has added 300,000 ounces to its assets under management so far this year, amounting to almost 6% of its total holdings. Combined with bullion-price gains for platinum, assets under management in dollar terms at the ETF have grown 10% so far in 2013. For ETFS Physical Palladium (PALL 0.89%), the same trend is apparent, with gains of 550,000 ounces so far this year representing about a 7.5% rise, and favorable price movements bringing the jump in dollar terms to about 11% year-to-date.

The better way to buy gold?
The figures above suggest that selling in metals ETFs is limited to gold. But even within the yellow metal's niche, there's conflicting evidence about whether gold ETFs or gold mining stocks are the better buy.

So far this year, mining stocks have continued their long streak of underperformance, with the Market Vectors Gold Miners ETF (GDX 1.91%) down more than 20% year-to-date. High costs of mining operations have squeezed margins recently, putting more pressure on miners than even falling gold prices would warrant.

Yet yesterday, gold mining stocks rebounded sharply even as the metal itself continued its lackluster performance. With the miners ETF rising more than 4% compared to just a half-percentage-point move for SPDR Gold, investors appear to believe that mining stocks represent better value at current levels.

A premature conclusion
In general, metals ETFs don't look like they're going out of style, but rather are seeing normal flows among different types of funds. Short-term fluctuations in assets will happen, but as long as the Federal Reserve's low interest rates make holding precious metals relatively cheap, metals ETFs will likely remain popular investments.