Have you actually moved away from the initial questin (NAS or HTPC?) or are you just a little confused still (no irony in the statement, just want to help)?
Anyhow, let's get the basics straight:
Media player/HTPC
- Provides playback of media
- Does not need necessarily provide any kind of storage
- Can be turned on-off/suspended-awakened freely, as the user is at the machine when it's needed
NAS or File/media server
- Difference is mainly if it's based on a full OS or not, what they have in common is the function to share files across a network, hence "Network Attached Storage" (as opposite to computer attached)
- Technically, a NAS has some kind of OS, but it's usually in the form of what could (simplified) be referred to as Firmware, i.e. an OS and file sharing application bundled. A NAS can basically only do one thing, namely share files, although some NAS software for DIY hardware may have more features
- A server is based on a full OS and can have multiple roles outside of file sharing. If you share a folder on your office computer, it basically turns it into a "server" (and the computer fetching the file thus a client). The term server is however usually referring to a machine which is configured to exclusively provide one or more server only roles.
- A full fledge computer running file sharing software (Unraid, FreeNAS) or a Windows computer sharing files over the network can be seen as a kind of hybrid, since the former are more competent than the over the counter consumer/SoHo NAS's (ReadyNAS, Qnap etc)
- In addition to it's file serving role, many/most NAS/servers also provide various kinds of data reliability, most often by various implementations of RAID (also see unRAID, which is quite popular in the XBMC community)
- NAS's and servers typically need to be powered on 24/7, since one wants to be access them from any computer on the network, without having to walk over to the machine for power on and then also wait for boot
For various reasons, people do however choose to build a combined machine for data storage and HTPC, but unless it also shares files to other computers/HTPCs on a network, that's then rather a combined HTPC and local storage machine, than HTPC and NAS/file server.
In my view, there's only one way to go if wanting to do it properly, namely separate playback from file storage. That allows for a low power, small size HTPC (which may lack storage capacity altogehter and boot from a USB, or take the shape of a WDTV or ATV) which can also be turned off/suspended when not in use. A NAS or file server then provides file access and streaming to one or more playback devices, allowing for good flexibility.
If I were you, and since you already seem to have the cabling in place, I'd buy and set up a low power HTPC in the living room and lounge and a NAS or server in the office. Heck, data reliability aside, you could just slam an extra HDD into your office PC, set up a shared folder and be done with it part 1.