Hi Michael,
If the keyword relation is just one to one, then an Autofill Keyword Set is generally the way to go.
Cascading Data Sets can do this too but their real benefit is when you have a data set that is dependent on the value in another data set. As an example, if you had State and City keywords, when you select the State, the City keyword's data set would only show cities in that state.
From an efficiency standpoint, unless you need an actual data set for the Group # keyword, the Autofill Keyword Set would be more efficient in this case as it would require less queries to the database.
Hope that helps.
Ansley
