To the best of my understanding the major difference is that when you go to record no 2 from record no 1 then the processing occurs on the client side and in the regular datasource it goes to the server side to find the values.
For eg in image gallery to find the subsequent image id it will have to goto server for the finding the value but on the client datasource it is available on the client side only.
In case I am wrong, I would take this oppurtunity to clear my concepts on the same.