What is the best way to import Accounts ONLY: PeopleImport or DemandTools?
Both will work. The chosen method is more a matter of preference vs. best practice.
PeopleImport is designed for adding/updating Contacts associated with Accounts.
However, it can be fooled by creating a column in the input file for Last Name and populating with "Unknown: Dummy for PeopleImport" or something similar which will not actually match to any Contacts. For the Contact match step, match on Last Name. Add one or more "New Contact - Existing Account" matching steps and set up the Account match criteria.
When creating a new Contact in an existing Account, any Account fields mapped in Stage#1 will be updated on matching Accounts per the mapping options.
In Final Matching Conditions select "Create New Contact/New Account".
Once the import is complete use MassEffect to export all Contacts where Last Name equals "Unknown:Dummy for PeopleImport" and created date = TODAY. Then use MassEffect to mass delete them.
DemandTools (Find/Report ID's and MassEffect)
Use Discovery -> Find/Report ID's in DemandTools to perform the matching to the Account Table. Then use MassEffect to update the existing Accounts and insert the new Accounts (two passes in MassEffect).
More information on Find/Report Id's can be found here.
Sr. Customer Success Manager