Main differences, 2011 vs 2012:
sandy bridge vs ivy bridge
ddr3 1333 vs ddr3 1600 ram
usb 2 vs usb 3
hd graphics 3000 vs hd graphics 4000
2012 had more ram on high end versions, and bluetooth 4 vs 2.1
If it were me, and the prices are similar, I would go for the 2012.