Exchange Traded Funds (ETFs) are a hit with investors, but what about the companies behind the ETFs? Dozens of companies offer ETFs, but the industry is dominated by asset managers Blackrock (NYSE:BLK), with its iShares brand ETFs, State Street (NYSE:STT), with its SPDR brand ETF offerings, and Vanguard (not publically traded) with over 70% marketshare. 

You may not have heard of Wisdom Tree (NASDAQ:WETF); it's a much smaller ETF provider , but it stands out as a pure-play, ETF focused company that doesn't participate in any other asset management or bankings services. Which of these three companies is performing best

Product Mix
iShares is a clear leader in this category offering a staggering 320 different ETF products. There are 141 SPDRs and 55 Wisdom Tree funds. All three have a wide range of region and industry specific options, but iShares, with its huge product portfolio, offers so many more targeted offerings. iShares is particularly strong in country-specific equity ETFs by offering targeted funds for over 20 different countries outside the United States.

Advanced investors and traders with country specific strategies in mind are, almost by default, going to use iShares ETFs.

Wisdom Tree has the most interesting and unique product mix. It doesn't offer any of the market cap-weighted ETFs investors are used to. Instead, Wisdom Tree has embraced the new trend of "smart beta." Smart beta ETFs are funds weighted by fundamental factors instead of market cap, and they aren't built to track a common index.

For example, Wisdom Tree offers a dividend growth ETF that contains 300 stocks with good three year track records of return on equity and return on assets. Then the fund is weighted not by market cap, but based on the dividend each company is expected to pay out.

Assets Under Management (AUM)
Although State Street owns the largest ETF by assets in the market, Blackrock is the clear leader in total ETF assets managed. At the end of 2013, Blackrock managed over 900 billion in assets in iShares alone, easily dwarfing State Street's SPDRs and Wisdom Tree's funds. 

Interestingly, Wisdom Tree, with its relatively limited product offering, is trouncing the others in AUM growth. Since 2011, Wisdom Tree's has a 55% compound annual growth rate in AUM. Blackrock's iShares comes in at 24% over the same period with State Street's SPDRs grew at 20%.

I expect iShares and Wisdom Tree to continue to lead the way in AUM growth because each have unique and differentiated products that will continue to attract new investor funds.

Foolish Final Thoughts
All three companies I looked at offered great choices for investors. While the State Street was no slouch, I came away most impressed with Blackrock and Wisdom Tree. Blackrock takes the crown for having the most products while simultaneously keeping fees low. Wisdom Tree may not have the huge portfolios of the others, but there is clearly demand for ETFs that take other factors into account besides company size, and Wisdom Tree is filling that market. 

So which one is a better investment opportunity? Depends on what you're looking for.

iShares, at just under a trillion dollars in assets, is less than a quarter of Blackrock's large and diversified global asset management business.  Wisdom Tree is the small fish in this big pond. The company is staking everything on its ability to do ETFs differently and better than the rest of the industry.

A defensive investor probably prefers Blackrock while the enterprising risk taking investor probably prefers Wisdom Tree. For now, I'm on the sidelines keeping an eye on both of them.