I see Tableau Prep (or any other ETL tool) as a mechanism to allows users to prepare their data without having to manipulate the source system.
In general any of your two proposals will work. But if you can do all the work in a single tool, why touch the source? Unless there is a huge performance gain, I would do all the ETL on Prep, including replicating your models, the data massage and necessary transformations.
But that is basically my opinion as a Prep user that has no access to manipulating the sources. Maybe other sys Admins will differ on this.
Note: you can always do a AB testing. Test one of the solutions, check its performance and then replicate the solution on Prep and see if it is compatible.
I hope this helps