Hardware upsizing. 1 15K rpm drive or 2 10K rpm drives?

I read on the todo list that 7.2 will have some form of indicating
different locations for certain files.

Would it be better to have two 10K rpm SCSI drives to distribute the data
around or would a 15k rpm be a better bet?

I am "upsizing" a database currently running on a 2 IDE drive Raid 1
(mirroring). I am letting go of the Raid 1, but after some discussion with
my boss we are only going to use PostgreSQL for generating some reports so
the data will always exists somewhere else.