If you buy new.... you know what you have.... and as long as you own it... you'll know the real history
If you buy used.... you really don't know. You may get a service history... and you may get a pile of paperwork.
But you'll never really know, the quality of the work... until you open things up.
It will always be your choice... a left over model, will fetch a lower price than a new bike... and most used bikes for sale (at least here in Raleigh) seem to be low mileage cream puffs...so there's good savings to be had.
Unless it's a bike that gets ridden often, and has the miles....They can't compete for the big bucks, and a buyer should think in terms of expensive/extensive repairs/replaing worn out parts..... but they might be the better deal (IMO) a lot of times