raid10 hard disk choice

Hello,
    i have to buy a new server and in the budget i have (small) i have to select
one of this two options:

-4 sas 146gb 15k rpm raid10.
-8 sas 146gb 10k rpm raid10.

The server would not be only dedicated to postgresql but to be a file server,
the rest of options like plenty of ram and battery backed cache raid card are
done but this two different hard disk configuration have the same price and i am
not sure what it is better.

If the best option it is different for postgresql that for a file server i would
like to know too, thanks.

Regards,
Miguel Angel.