more 10K disks or less 15K disks

I am faced with a hardware choice for a postgres data warehouse
(extremely high volume inserts..over 200 million records a day) with a
total storage of either

12 x 600G disks (15K) (the new Dell Poweredge C server)

or

24 x 600G (10K disks)

ALL direct attached storage.

I am leaning toward the 24 disks as I expect the higher number of
disks to provide overall better performance under high loads

Does anyone have any experience with a mixed 10K / 15K DAS storage
that you can share.

Thank you

Sriram