I would say a SAN or NAS over a DAS, because to model DISK IO is a little hard.

I would look for a system that will let me expand a raid group or create a raid groups and expose it to the OS with out down time.

Let say you start with a raid group A 1_0, 20 drives, 15,000 RPM SAS, fiber. 10 disk are available for writes and 20 disk available for reads. You find your self with bad performance. No or limited maintenance windows. Also who want to work till 4 in the morning? A SAN or NAS will let me expand the file group I might add 6 disk to my raid group A 1_0 to increase IO or I might create a new file group and move some tables or indexes over to it. Balance my IO. Flexablity means cost. I would look at NETAPP NAS (cheap) or Go with a IBM DS8000 ( Expensive).

SAS Drive is design to run for 5 years where SATA is an upgrade to PATA drive and is designed on average for systems that is not running all day.