As of our Spring '18 Release, we enhanced the BOM comparison to another item.
Go to an Item and Compare to Another Item.
Search for an item and Select the Item to Start Compare.
On the Compare BOMs dialog, you can switch to a different revision of the compare to item or select another item.
Children shared by both comparison items are highlighted in green. Differences are highlighted in yellow. Rows without highlights are only in one of the comparison items.
You can also export the comparison data to CSV or XLSX formats.
If there are additional comparison fields you want to see, contact your Salesforce Administrator. The comparison fields are controlled by an Assembly object field set called "Compare To Another".