Out of experience, I'd go for the disks that give you the highest continuous write throughput. Most of the time, SAS does sequential reads and writes on datasets; random access happens mostly in utility files created when you do complicated SQL joins/operations.
SSDs have the big advantage that they don't create latencies when concurrent use (or SQL, as noted above) forces random accesses; if you can't satisfy your I/O needs with one drive, get several and create a striped volume over them. Also keep WORK/UTILLOC volumes physically separate from the rest of your storage.
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