These guys have it covered pretty much. There isn't much difference and other than native USB 3.0 support, you probably won't notice any difference going to the new platform. If you have the bug to upgrade, either go Socket 2011, or wait until June and check out Haswell. If you are on a budget and want to get the most use out of a rig, the next logical step from Sandy Bridge is whatever comes after Haswell. If you DO have the money, and it is burning a hole in your pocket, build a Haswell rig. If I'm not mistaken, Sandy Bridge rigs are creeping up on 2+ years old and honestly will be good for that much again. You have to decide how much mileage you want to get out of your rig. I went from an AMD Athlon 64 X2 rig to an Intel 3570K Ivy Bridge rig........because IB had just came out and it was the logical upgrade path. I've never recommended anyone who currently has SB to upgrade to IB.
Short version: If your current SB rig is handling everything you throw at it, get more mileage out of it. Upgrade when there are significant advances in hardware, such as DDR4, SATA Express, or PCI based storage......which will be out before you know it.