I don't have the Campbell-Stokes but do have the Blake-Larsen
http://www.sunrecorder.net/the-history/The attached screen shot compares the Davis VP2 Plus as calculated by Cumulus (using default settings so no tweaking done) vs the B-L.
I have compared the log hours from Weatherlink, CumulusMX and Blake-Larsen over the past 15 days:
WL CumMX B-L
11.0 8.8 9.15
9.5 3.8 5.30
2.8 0.0 0.00
5.5 0.1 0.00
7.7 0.0 0.10
10.8 5.2 7.54
1.2 0.6 0.00
9.2 7.1 7.84
0.7 0.1 0.00
10.8 9.4 10.14
9.3 2.7 2.94
7.2 0.6 1.38
10.7 2.7 2.94
9.0 2.8 3.22
10.0 5.0 5.63
The Weatherlink seems to be way off, and that could be because I run Weatherlink only periodically, every few days, and the extraction from the logger could throw the calculation off.
CumulusMX Follows the B-L reasonably close but shows sunshine hours when there were none.
The B-L is quite precise as can see the sunshine off or on in a matter of seconds as the sky changes.
Our local airport is one of the location where Environment Canada no longer records bright sunshine as they are not replacing aging equipment (Campbell-Stokes).
Enjoy,
Paul