Exchange traded fund investors have a number of options to track the S&P 500 Index, but each offering show small differences that can produce different results.

For starters, the SPDR S&P 500 ETF (NYSEArca: SPY) is the largest ETF and the first U.S.-listed ETF to hit the market. With about $20 billion in shares exchanging hands on average each day, it trades more than any other security, has the most liquid options market of any ETF and tight bid-ask spreads, according to Morningstar analyst Michael Rawson.

All of these attributes “make this ETF ideal for institutional traders,” Rawson said.

However, potential investor should be aware that SPY is structured as a unit investment trust and not a regulated investment company like other funds. Consequently, the structure prevents SPY from reinvesting dividends, holding securities that are not included in the index, like futures, or engage in securities lending. Moreover, the ETF has a one-month lag beween the ex-dividend date and the payment of its dividends.

“These factors have caused SPY to lag its index by more than its expense ratio, particularly in bull markets,” Rawson added.

Specifically, SPY has returned an annualized 17.8% from March 1, 2012 through February 28, 2015, compared to a 17.97% return from the Vanguard 500 Index (NYSEArca: VOO) and 17.93 from iShares Core S&P 500 ETF (NYSEArca: IVV).

VOO and IVV have also been quickly gaining traction as an alternative method for accessing the S&P 500.

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