Investors who buy mining stocks often do so as a leveraged bet on silver prices. It is little wonder then that central banks were a key driver of silver miner ETFs' strong performance in the third quarter. Though current market conditions may have obscured the memory a bit, at the beginning of the last quarter silver was in the red and investors were on the sidelines. In August, there was a notable shift in sentiment and investors returned to play.

Anticipation that the European Central Bank and the Federal Reserve would intervene in the markets and the generous programs that confirmed these expectations not only turned the tide among physical investors, but also among silver miner ETF investors. The rewards were more attractive for the latter. According to ETF Trends, SIL was a top performer among all types of ETFs in September.

SIL is a much larger fund than SLVP, which has a market cap of only about $2.38 million. The larger ETF has been around since 2010, but SLVP was introduced to the market earlier this year. While investors are clearly showing a preference for SIL, eight of the top 10 holdings in both ETFs include the same companies, but represent different percentages.

Both funds also provide exposure to the full gamut of silver companies. That may initially be a turn off for some investors since junior miners have a reputation for extreme volatility that many find unappealing. Some market veterans even warn that the junior mining sector is unsuitable for most silver investors.

But there are two things that increase the appeal of silver miner ETFs for those who want to invest in silver companies. First, advisers admit that when good selections are made, junior miners can offer highly attractive returns. Second, the way these funds weigh juniors compared to how they weigh larger companies significantly reduces risks while still providing diversity.