'In order: New > Refurbished > Open Box > Demo. For open box, as long as you can see the TV beforehand (no scratches, no burn ins, etc), it should be fine. If the savings are substantial, go for it.'
I would put it in this order; New, Demo, Refurbished, Open Box. Demo models can be great bargains. Just be sure to buy from a dealer that takes care of their products on display. The time on display is a good burn-in period and will reveal any weak points, such as a bad power supply.
Refurbished can be good, but you never know until you get it. Open box means it was returned, and often people don't disclose a fault to the dealer.
I recently bought a Samsung UN60H7100 display model from Costco and it's great. I got it for $800 and Costco gives you 90 days to test it out and a two year warranty. Plus, Costco offers the Square Trade five year warranty for $60.
I've sold a lot of display models over many years when I worked in retail electronics and I can tell you that the display models had a lot lower failure rate after purchase because they had already gone through the burn-in period.