Although the two report are similar they were written for different purposes.
Inventory Audit Report:
Found under Inventory->Inventory Reports->Inventory Audit Report
This report is more robust and has a lot information than the inventory posting list. This is the only report in the system that will, and is meant to, tie your financials to your inventory.
This report is meant to represent the inventory valuation for financial purposes.
Inventory Posting Report:
Found under Inventory->Inventory Reports->Inventory Posting Lsit
The Inventory Posting List is meant to track your inventory movement (i.e. the ins and outs of your inventory). You can also use different valuations to make what-if reports
This is more referential and shouldn’t be used to reconcile against the inventory financial value.
The bottom line is if you want to track your inventory, both will work, but the inventory audit report is the one you should use if you need financial information as well.