The biggest difference between the Sandy Bridge and the Ivy Bridge cpu is the TDP.
The Sandy Bridge will have a TDP OF 95W while the Ivy Bridge will have a TDP of 77W so that makes the Ivy Bridge a lower power cpu. Lower power means less heat and so the Ivy B. cpu will do the same work a little faster and generate less heat and that makes the I.B. a better choice then the S.B. for your laptop.