The adafruit forums have a topic for laser cutters and laser cutting businesses.
http://forums.adafruit.com/viewforum.php?f=18
I would expect 9without any direct experience) that there would be significant advantages to the level of support that comes with a new machine, unless you can't afford it. If you already have a lot of experience, or if this is your second laser cutter, or if you CAN'T afford a new machine, then a used machine becomes more attractive.
If you have a "hackerspace" in your area with a lasercutter (TechShop, sawdust shop, etc) it is my observation that you can buy a LOT of time at such a place for the cost of a new laser cutter...