The best reference for answering your question is the BOL topics for each of them:
OPENROWSET
http://msdn.microsoft.com/en-us/library/ms190312.aspx
OPENDATASOURCE
http://msdn.microsoft.com/en-us/library/ms179856.aspx
From personal experience OPENROWSET can do what OPENDATASOURCE can, but it can also read text files using format files, and different code pages and perform BULK operations where OPENDATASOURCE is not as flexible in the aspect.