NetappDow -
As Yishikawa mentioned, it's a business requirements question.
RDMs are a bit more work - creating the LUNs, mapping, and mounting them.
VMDKs are easy to work with.
RDMs may yeild better performance.
A typical use case would be with high performance databases.
SATA drives are often used for backups, or light IO storage.
The metric I use to determine SATA v SAS provisioning is IO density, expressed as max random IOs/TB of data.
( ~300 for your SAS drives and ~50 for your 2T SATA drives )
I hope this response has been helpful to you.
At your service,
Eugene E. Kashpureff, Sr.
Independent NetApp Consultant http://www.linkedin.com/in/eugenekashpureff
Senior NetApp Instructor, IT Learning Solutions http://sg.itls.asia/netapp
(P.S. I appreciate 'kudos' on any helpful posts.)