Looking at other messages regarding these two tools I think this is the idea:

• The data migration tool is the more “old” way of migrating data and will be phased out
• The new way is using the Data migration manager and will become default in IFS

So based on this reply 

It looks like it is good to start to migrate to the Data migration manager (or old name Smart data manager) 

There is a course on IFS Academy and you can find some information about it in the Technical documentation

https://docs.ifs.com/techdocs/22r1/030_administration/050_data_management/080_data_migration_manager/

Hope this helps!