Good question, I was talking to a few guys at work about this same subject.
Over the last few years, I've been one to check both for future (5+) days forecast for home and vacation spots that we were going to. My opinion is that accuweather seems to do a better job of predicting the forecast (espically rain) for the future. For example, a number of times I've checked and weather.com would show rain (and a high %) where accuweather would say chance of rain, or partly cloudy with a smaller chance of rain. When the weekend or trip comes up, things were not as 'bad' as weather.com indicated.
I've not done a scientific study, but may some day. Something create an excel spreadsheet of the 15day forecast and update it every day, then recording the actual weather.
So the short answer for me is check both, but things (rain) isn't as bad as weather.com would indicate...