Besides the fact that both are extremely sophisticated SaaS there is nothing in common in terms of product. Salesforce is a CRM system and Workday an HCM and ERP system.
If you want to establish comparisons "apple to apple" I would recommend to compare Salesforce against Oracle Sales/Marketing cloud, SAP and Microsoft Dynamics 365
On the Workday side I would compare it with:
a) HCM - Oracle , Bamboo HR, ADP workforce Now, Ascentis and SAP HCM
b) ERP - Oracle ERP cloud, Netsuite (owned by Oracle), SAP, Microsoft Dynamics 365 Business Central and Sage.
Hope this helps