BBU Cache vs. spindles

I'm weighing options for a new server. In addition to PostgreSQL, this
machine will handle some modest Samba and Rsync load.

I will have enough RAM so the virtually all disk-read activity will be
cached. The average PostgreSQL read activity will be modest - a mix of
single-record and fairly large (reporting) result-sets. Writes will be
modest as well but will come in brief (1-5 second) bursts of individual
inserts. The rate of insert requests will hit 100-200/second for those
brief bursts.

So...

Am I likely to be better off putting $$$ toward battery-backup on the
RAID or toward adding a second RAID-set and splitting off the WAL
traffic? Or something else?

Cheers,
Steve