Refurbished will be the better option most of the time. In the UK, the term isn't even used in the way that many assume it to be. Products described as refurbished are often no-fault customer returns (rather than items that have been repaired), that have had minimal - if any - usage. The refurbishment process simply involves checking the product works as it should.
Manufacturer refurbished products can be good too. As an engineer once pointed out to me, in some circumstances it can actually be preferable to buy a device that has been screened. Hardware and software will often be updated to remove issues found in initial production runs.
It's all a question of what kind of discount your are getting. Whilst refurbished products can have their benefits, buying new is still generally better. So there is needs to be a decent reduction offered to make purchasing a refurbished product seem like good value.
As for used products, I wouldn't pay as much as I would for the refurbished equivalent. With used purchases you are often taking a greater risk, as typically minimal maintenance has been carried out, and you won't generally be offered a warranty.
That's not so say there aren't some used bargains to be had, though. This is can be especially true when buying from someone you know personally, or the seller has a good reputation.