Neither will give you a PERFECT answer. Constant returns and inflation in forecasts don't INCLUDE the effects of a retiree having to make a withdrawal in a down market year. In contrast, Monte Carlo computer models vary returns every year using statistics of the past in numerous iterations. Some vary inflation too. Monte Carlo analysis gives a “success probability” assuming that the statistics of the future will be the same as the past, something that many forecasters believe doubtful.
William Bernstein, a highly RESPECTED ANALYST and author, suggests using no more than a 6.5% return for equities and 3% for bonds. With results from my own programs, I lean toward using the actual returns and inflation for each year starting with 1965 because I feel we will face similar conditions with periods of high inflation and serious volatility.
Since none of us can really forecast the future, what’s important is to do a new forecast each year using the most recent balances and future events as you see them at the time to adjust your savings or spending accordingly. Future events should include the need for some long-term-care and death of a SPOUSE with consideration to survivor benefits.

Neither will give you a perfect answer. Constant returns and inflation in forecasts don't include the effects of a retiree having to make a withdrawal in a down market year. In contrast, Monte Carlo computer models vary returns every year using statistics of the past in numerous iterations. Some vary inflation too. Monte Carlo analysis gives a “success probability” assuming that the statistics of the future will be the same as the past, something that many forecasters believe doubtful.
William Bernstein, a highly respected analyst and author, suggests using no more than a 6.5% return for equities and 3% for bonds. With results from my own programs, I lean toward using the actual returns and inflation for each year starting with 1965 because I feel we will face similar conditions with periods of high inflation and serious volatility.
Since none of us can really forecast the future, what’s important is to do a new forecast each year using the most recent balances and future events as you see them at the time to adjust your savings or spending accordingly. Future events should include the need for some long-term-care and death of a spouse with consideration to survivor benefits.