XenServer has two processor, but only one is visible

It depends on your load. If your app is single-threaded, CPU-bound one, the GHz trumps almost anything. If you have a multiple multi-threaded apps (e.g. a web server), then core count is more important than single-threaded performance, and if you are I/O bound, then CPU doesn't really matter much.

There is a lot more to your question than just cores and performance.

• How many concurrent users does your server need to support?
• Are you with one server and no redundancy? What is the acceptable downtime for your application?
• Have you done any benchmark of your development machine for this application to somewhat extrapolate performance?

You may be too worried if you do not expect too much traffic. If you want to put all your apps on a single server, you risk complete outage if hardware has issues. See if you can go for 2x lower machines and distribute the load. For performance, you can dedicate cores to PostgreSQL process using taskset so DB performance is manageable.

If you manage your disks well, then you may get better performance as well. For example, set 2 disks in RAID 1 for pg_xlog.

The long answer is, benchmark your application, and consider redundancy if you cannot afford downtime. Also, compare costs with cloud solutions which will help if your application can scale.