A group of independent wealth managers has been told by market historian Jeremy Siegel that stocks are a much better buy than other asset classes, particularly U.S. Treasury securities.

Siegel, a noted professor at the University of Pennsylvania's Wharton School, said in his well known book for investors, 'Stocks for the Long Run,' that there should be more exposure to international equities.

As of Friday, the S&P 500 index was sporting a price-to-earnings ratio of 15 times this year's estimated operating earnings and 21.8 times estimated reported earnings.

The index's operating earnings yield, a tool for comparing equity valuations with bonds, is 4.6 percent, topping the 3.9 percent yield on the 10-year U.S. Treasury note and matching the 30-year bond's yield.