Conventional wisdom dictates that investors are usually better off emphasizing stocks in their portfolio, with bonds utilized mostly as some protection if the stock market falters. Certainly recent numbers seem to back up this position, as the Standard & Poor's 500-stock index ended August up 51%  from its March low, for a 13% return year-to-date up to that point.

But financial experts are now arguing that it's time to bring bonds back into the picture, for several reasons. First, stocks are obviously not as cheap as they were just a few months ago. Next, many foresee a slow recovery in earnings over the next few years. Even if stocks do a little better, they are subject to higher volatility.

"What's more, the classic 60-40 split between stocks and bonds-the formula that many balanced funds use to allocate investments-ignores alternative asset classes that can deliver returns with different levels of risk," notes an article in MarketWatch. Many financial planners no longer stick by the traditional 60-40 split.

Over the past year or two, stocks haven't done well. The S&P 500 is still down 30% since Dec. 31, 2007. And what about bonds? The Barclays Capital U.S. Aggregate Bond Index is up 10.1% over the same period.

Certainly many have held that over the long-term, stocks are a better bet. However, points out MarketWatch, bond investors are also ahead of stock investors over the past five and 10 years. "Over the past five years, the S&P 500 returned an average 0.1% a year, while the Barclays bond index returned an average 5%. Over the decade, the stock measure fell an average 3.7% a year, while the bond gauge returned an average 6.3%."

Historically, meanwhile, stocks have had higher highs but much lower lows than bonds. In their best year in the past half-century, 1975, stocks delivered a 37% total return-but they fell 37% in their worst year, 2008. By contrast, the best year for bonds was 1982, when they produced a 36% total return, while their worst year was 1999, when they fell 6%. In addition, many advisors now say that asset allocation plans should include a significant, perhaps 20%, dose of alternatives like commodities.

Yet, there remain a persuasive body of expert opinion who favor stocks. The article cites Ned Notzon, chairman of the asset-allocation committee at T. Rowe Price Group Inc. He holds that stocks will generally beat bonds over long time periods.