which hardware setup

I work for a web app to send email newsletters, and I have one question about postgres' performance in two different setups. Actually we have one 4GB Ram VPS running our app server (it's a rails app under nginx and thin) and a 4GB Ram VPS running the database (18GB). We want to migrate to bare metal servers, but I have made two different setups and don't know what is the best option:

Option 1:

App Server: Dual Xeon 5130 dual core with 4GB ram and SATA disk 

Postgres: Xeon 3360 quad core with 4GB ram and 2 x 146GB 15k RPM SAS (RAID1) disks 

Option 2:

App Server and Postgres: Dual Xeon 5520 quad core with 12GB ram and 2x 146GB 15k RPM SAS (RAID1) disks

I know the first option would be better in terms of I/O for postgres, but our app server doesnt use much I/O and with the second option we would have much more ram.

Thank you

Pedro Axelrud
http://mailee.me
http://softa.com.br
http://flavors.me/pedroaxl