Is investing in index exchange-traded funds (ETFs) boring? Not if you want to build a fortune over the long term. Actually, an S&P 500 index fund offers a great way to retire as a millionaire.

Two S&P 500 ETFs -- SPDR S&P 500 ETF Trust (SPY 0.63%) and Vanguard 500 Index Fund ETF (VOO 0.62%) -- are by far the most widely traded. But there's another ETF that isn't as well known that you might want to consider. Could this alternative S&P 500 index ETF make you more money than the popular ETFs?

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A weighty subject

The alternative I'm referring to is the Invesco S&P 500 Equal Weight ETF (RSP 0.88%). Like SPY and VOO, RSP allows you to own every stock in the S&P 500. 

The standard S&P 500 index that SPY and VOO attempt to track is weighted by market cap. In other words, the stocks with the greatest market caps such as Apple and Microsoft make up a greater percentage than stocks with lower market caps.

Invesco decided to take a different approach with its RSP fund. The S&P 500 index it uses is equal-weighted. The smallest stock in the S&P 500 based on market cap receives the same weighting as the biggest one.

Sure, the weightings shift somewhat as the market caps of the S&P 500 members rise and fall. However, the S&P 500 Equal Weight Index and the RSP ETF are rebalanced on a quarterly basis. 

Delivering higher returns

SPY, VOO, and RSP are down over the last 12 months. But RSP has performed a little better than its two peers.

SPY Chart

SPY data by YCharts

The story for RSP looks even better over the last three years. The equal-weight S&P 500 index ETF has handily beaten the two more popular S&P 500 ETFs.

SPY Chart

SPY data by YCharts

Looking at total returns makes RSP appear even more impressive. Its yield is higher than the yields of SPY and VOO. This boosts RSP's total return, giving it a bigger advantage over the last three years.

SPY Total Return Price Chart

SPY Total Return Price data by YCharts

Note that these total returns don't include expenses. Invesco's expense ratio for RSP is 0.2%. That's higher than the 0.0945% and 0.03% expense ratios for SPY and VOO, respectively. However, this difference still isn't enough to offset RSP's bigger returns. 

Recency bias

There's no question that RSP made more money for investors over the last three years. Will this outperformance continue in the future? Maybe, but maybe not.

If we only compare these S&P 500 index ETFs over a three-year period, RSP wins hands-down. It's a different story, though, if we go back further. RSP trails SPY and VOO over the last five-year and 10-year periods.

Recency bias is a cognitive bias where people focus more on recent events than they do on the longer term. Investors shouldn't succumb to recency bias when choosing where to put their money. There's no guarantee that RSP will be the better choice going forward.

I suspect that RSP won't be able to beat SPY and VOO over the next decade. Why? My view is that the increased adoption of artificial intelligence (AI) will provide a huge tailwind for Amazon, Alphabet, and Microsoft, all of which have cloud businesses that should profit from the AI boom. I also expect that Apple will benefit from AI and the rollout of 6G networks, which some experts predict will be on the way by 2030.

These four companies combined make up roughly 17% of the portfolios for SPY and VOO. If they outperform the overall market in the coming years, these two S&P 500 index ETFs will likely beat RSP's returns. Regardless of which S&P 500 index ETF you choose, though, I think there's a good chance you'll make plenty of money over the long term.