• There's "no way" bonds are better than stocks for long-term investors, Wharton professor Jeremy Siegel said.

• By the time investors have doubled their money in bonds, they've quintupled their money in stocks, he estimated.

• Siegel has turned more bullish on stocks and the US economy over the past year amid inflation's decline.

Bond yields are attractive, but there's "no way" they beat stocks for long-term investors who want to build their wealth, according to Jeremy Siegel.

The Wharton finance professor pointed to rising bond yields over the past year, with the yield on the 10-year Treasury note trading around 4.34% on Monday, the highest in 16 years.

And the yield for Treasury Inflation-Protected Securities is currently hovering around 2%, according to Federal Reserve data. That means it will take around 36 years to double your cash in TIPS, Siegel said.

Meanwhile, the S&P 500 is currently trading with price-to-earnings ratio of about 20, which means stocks have around a 5% yield. It will take investors around 14 years to double their money in the benchmark index, he estimated.

"By the time you've doubled your money in bonds, you've multiplied your money by five times in stocks," Siegel said in an interview with CNBC on Monday. "People tell me, 'oh my goodness, bonds are as good as stocks'. No way for long-run wealth creation."

Siegel has turned more bullish on stocks and the US economy in recent months, despite being one of the first on Wall Street to sound the alarm for a recession in 2022 that has yet to materialize.

But his view for a potential downturn has changed, he said, thanks to falling inflation and a resilient economy that's been propped up by a boom in productivity.

Besides a few months surrounding the pandemic, the last quarter was the best quarter for productivity in six years, Siegel estimated. That's good news for the US, as it means the nation's economy can see strong GDP growth without stoking inflation.

"This is really saving [Fed chief] Jay Powell. I mean, this is the best news," he added.

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