When it comes to everyday computers, I always recommend buying new. You'll get a fresh system without wear plus you'll have Apple's factory warranty intact. When you buy a used machine, you never know how it was used by the previous owner. Think of this like a used car--sometimes the car was driven every other Tuesday for a 5 mile round trip, sometimes it was driven on a 27 mile commute through rush hour by an individual with a lead foot on a daily basis. The same applies for computers--components such as hard drives may have been used excessively over time and thus have a greater chance of failing earlier.
It's fine to buy an extra computer (i.e. not for daily use) or vintage computer used under most circumstances, but please be aware of the risks of purchasing from third party sellers. Think of a refurbished Apple machine as a "certified used car"--it's been inspected and has a lesser chance of failure, but the components have still accumulated wear and tear not present in a new model.
Although the extra screen size is tempting, the 21.5" should be adequate for most purposes (although there are some exceptions--what are you planning on using this computer for?) and will likely fit on your desk without a problem. The 27" model is almost too large for a home or student user, although there are again exceptions depending on the machine's expected uses.