If it's a good small car the depreciation is very little, I'd buy new. Lets take the Honda Fit for example.
New
2012 Honda Fit sport/Auto
MSRP $18,700
Lets say you don't negotiate too hard
$18,000
Used-Using cars.com in a 250 mile radius
2012 Honda Fit Sport-9,460 miles-$18,999
2012 Honda Fit sport-3,265 miles-$18,491
2012 Honda Fit sport-Miles not listed-$17,500
2011 Honda Fit sport-2,296 miles-$19,589-Guessing it has navi
2011 Honda Fit sport-17,200 miles-$17,500
2011 Honda Fit sport-25,984 miles-$16,998
2011 Honda Fit sport-25,907 miles-$16,449
2011 Honda Fit sport-36,447 miles-$15,998
2011 Honda Fit sport-14,133 miles-$15,997
2011 Honda Fit sport-27,281 miles-$15,879
So in summary, you are not saving anything or very little by buying a 2012 used. By buying a 2011 you're saving about $2-$3k.
So lets say that you save $2,500. When you go to sell the car it's going to be worth less because it's a year older and it's going to have an additional 20-30k miles. So between those two things that's going to reduce the difference by at least $1,000. So now you're saving $1,500.
Your 2012 may have more features. They likely added something and doubtful that they took anything away.
You may not know the history of the car. Did the person that owned it before ride the brakes? Did they change the oil? Did they rotate the tires? Did they get in a little fender bender that you'll never know? Do they have a teenage son that liked to do neutral drops at the stoplight?
Everything is 20-30K miles older. The brakes, engine, trans, interior, paint, etc all have 20-30k miles of wear.
Warranty. Are they going to add a 3/36mile warranty?
If you're financing are you going to be able to get a zero rate?
Like I said, I normally buy used cars but in some circumstances I'd rather just eat the $1,500 in savings and have peace of mind. Last year I bought a 2008 G35x w/25k miles, had a warranty until 100K miles, and cost $40k new. I bought it for a little over $25k. In that situation I felt like I could give up some of the things for the $15K savings.