I got a 2012 iMac about a week before the 2013 ones came out. From what I can tell from using 2013 models in an Apple store, the 2012 iMac doesn't seem any slower. 🙂
It actually depends on the configuration you're planning on getting and what you're using the iMac for. If you are getting an iMac with an SSD or Fusion drive, definitely go for the 2013. If you use WiFi on your iMac, consider the 2013 to have the faster WiFi available on it.