I spent less time re-jigging the P4, then I spent searching forums and reading reviews and tech info.
Space wise - much the same if you run headless.

A NAS can be a bit noisy, as they can use smaller fans, and smaller fans = more noise.

A few NAS units also don't power down the internal HDD's, and also have had problems powering down external USB drives as well.

A lightweight PC wont draw that much power, and if you use, for example in WHS, "Lights Out", WHS will automatically power up and down from hibernate / standby only when you are using it. Drops to hibernate when no network activity after 10 mins. Check out t he WHS forum on GZ for further details.

And WHS can use BT clients, TV recording, and all that guff.