What is the different between compression on the PK vs. on the Table?
It's not a matter of putting compression on a primary key, but instead a matter of putting the compression on the clustered index. For SQL Server, a clustered index is organizing the table's physical structure on that index. Or, in shorter form, the clustered index is the table. This means that compressing the clustered index and compressing the table are functionally equivalent. If you were to create your primary key as a non-clustered index and keep the base table as a heap, these two structures would be different and compressed separately.