Three main differences:
1. Ivy Bridge processor - slightly faster and cooler; potentially better battery life.
2. USB3 vs. USB2 - speeds up transfers, etc., and really speeds up Time Machine backups with an external USB 3 drive.
3. HD4000 graphics - notably better than HD3000 in the Late 2011.
Depends on the price difference - if it's only $50-150, go new. If it's $200 or more, I'd probably go used and then put Applecare on it.