I'd certainly second the idea of building your own and getting to grips with FreeNAS. That said, I've gone with both options:
I'm currently running 2 NAS boxes: 1) Thecus N4800 Eco with 4x 4TB WD Se drives in RAID0 and, 2) a home-built NAS (Silverstone DS380 chassis, ASRock C2750D4I mainboard, 32GB (4x 8GB) Crucial DDR3-1600 ECC UDIMMs, 128GB Patriot Torx 2 SSD for the boot drive and 8x 6TB WD Reds for the storage) running FreeNAS in Z2FS. The chassis is small enough on a desk that it's not too much of a hindrance although the 3 supplied fans can become noisy when the room's hot. The second NAS was really designed to be a backup of the first with redundancy and additional storage in mind.
So far I've not been disappointed with the build of NAS#2 and the Thecus has proved to be a trooper.
