One is a 60/40 critic: Ben Inker, co-head of asset allocation for GMO, a Boston-based manager of $104 billion for institutional clients such as endowments and pension funds. He also co-manages Wells Fargo Advantage Absolute Return (WARAX), a mutual fund that uses alternative strategies.Also weighing in is Fran Kinniry, who embraces traditional stock-and-bond portfolio construction. He's a principal in the investment strategy group at Vanguard, the nation's largest mutual fund company, managing more than $2 trillion. Below are edited excerpts of their arguments on whether a 60/40 portfolio remains a good tool for middle-aged investors trying to build up retirement savings: INKER: One reason I'm skeptical about 60/40 is that it's probably not aggressive enough, at least for a 40-year-old investor. You need to invest more in assets that are riskier than bonds if you want to meet your investment goals without having to save an extremely large percentage of your income. If you're 40, you've got around 25 years before retirement. That's a long time. With significantly more than 60 percent in stocks, you'll have a much better chance to achieve your retirement savings goals than you would with just 60 percent. Your main investment goal at that point in life should be trying to generate the highest return on your savings. While an aggressive allocation to stocks makes sense at that age, that doesn't necessarily mean you can rely on alternative assets to diversify a portfolio. The problem that a 60/40 portfolio presents is that you can't rely entirely on bonds as a diversifier, either. They have been good diversifiers in recent years because we've had low inflation. In fact, higher-quality bonds like Treasury notes have been especially good diversifiers. But if the future risk we face is from rising inflation, bonds aren't going to help. They'll be lousy diversifiers, and will hurt your portfolio.