All the sites mentioned above are using the Google Maps API to read the elevation data from Google. I haven't found out what the datasource is for Google. My only comment is that none of the websites based on Google are going to give you elevation gain accuracy that is close to what 24K datasets provide, such as DeLorme TopoUSA or Garmin Topo 24K. The problem is that the elevation points on google are too far apart. Between two points at the same elevation, you may have gone up and down, but as far as the dataset is concerned, you didn't change elevation.

The difference between the above sites is how they plot the routeline and calculate the elevation. I'd worry less about which one is more accurate and use the same site to compare climbing ride to ride.