I will not say the NWS is the site to use because they are more accurate rather I will say that if you want a good comparison use the NWS as your base line and which of the two more closely resembles the NWS reports.
As a Skywarn Spotter I use many resources for my weather reports including Weather.com and Accuweather
To fairly answer this question everyone needs to look at the National Weather Service as a baseline
because television, radio and other media weathercasters prepare their forecasts using NOAA information.
I understand that you want to use only Accuweather.com or Weather.com but you do need to use the NWS site to compare the two.
I have been watching the NWS, Accuweather and Weather.com and it appears to me that it is a toss up as to who is more accurate.
The reason why is that the NWS works closely with all media to work together to try and prepare there reports uniformly with the NWS. So more and more you will find that the accuracy of the reports will all closely resemble the NWS reports. The only difference is that media reports will be targeted to there customer base, but the accuracy will pretty much be in line with the NWS.
I hope this helps
See this link on Transforming the Weather Business
http://celebrating200years.noaa.gov/transformations/weather/welcome.html