As far as I am aware one of the key difference is "License Asset" module available under Portfolio is enabled by default and there is no charge linked to it and you can use it to track your license and entitlement info like Device or User allocation with number of License rights purchased against a model.
It is a basic version compared to what Software Entitlement provides which comes with SAM Pro plugin which is a paid version and involves additional tracking capabilities like Publisher part number, multiple license metrics available OOB for different Publisher which is not the case with the former available free. Also you can define your own custom metrics allows you to Reconcile the data based on information available in Software Entitlement and Software Discovery Model for better tracking if you are over spending, or any other compliance issue.
SAM software Entitlement allows more tracking with much more attributes
Also to note ServiceNow content Library keep in updating Publisher Part number info which is not the case with License Asset available by default.
Also when you enable SAM Pro License asset module available under Portfolio will not be visible as both are redirecting to the same table "alm_license" with different view and more tracking capability depending on which plugin you have in your instance.
Hope this helps. Please mark the answer as correct/helpful based on impact.
Regards,
Shloke
