I received an interesting call today from my father, who is currently looking for a new car.
It appears that the local dealership near them was getting a substantially lower value for his trade-in than he was expecting (20% or so). When asked, they indicated their source was Kelly's Blue Book. My dad had checked the value of his car using Edmunds just that morning before heading out and didn't understand the discrepancy, so he called me from the dealership and I checked it out for him.
Sure enough, using the same data on the two services resulted in a significant (20%+) delta in the prices, with the Edmund's prices being higher than the Kelly's prices.
I then decided to check out the same question with my cars. To my surprise, nearly the same result was achieved for both of them. Although the percentages were somewhat smaller than the percentage delta for my father's car, the Edmund's price was always higher than the Kelly's for the same information.
An interesting additional bit of information. The difference of opinion is primarilly in the trade-in value. If you look at the private party sale and retail used values, the two services are much closer to each other.