Decide what services you want to run on your NAS and what performance you will be happy with.

A single smbd (Windows networking copy) uses 60% CPU  30MB/sec on my TS-212 Qnap.

I run a TV DVR on my qnap with a USB DVB-T stick, works well.  Tried out PLEX media server and it maxed out the 1.6GHz ARM in my QNAP.

If its just file services and media serving without transcoding go for an ARM based NAS.

If you want the bells and whistles and have $600 for electricity a year to run a C2D 24/7/365 then go for an old PC.

Everybody's needs and standards are different.