Let me preface this as I realize that these are separate services and the pro's and con's of each. We have been looking at setting up one of the services but are more leaning towards a SUS as we need to control distribution of software OS software updates per our security group. That being said a caching server would be beneficial for us regarding non OS updates from the App Store as well as eBooks.
I am planning on testing to see if I set the software update distribution server on our clients if it will overwrite just the software updates that the SUS delivers or just all of the caching sever in general. I figure I would ask here to see if anyone had tried and what your results were. I have looked at these 2 Apple articles but they are not 100% clear on this either. About Caching Service, Content types support by caching service in OS X server
We also queried with Jamf and they also are not 100% sure either. I may be crazy for even wanting to do this but it is a thought and figured I would ask.