I dont see any reason to go for the third gen in terms of real world performance. I dont give a rats a** about heat or power usage, my power supply and liquid cooling will more than compensate for anything like that.
If you don't have a Sandybridge already it is worth it to go to Ivybridge. If you already have a Sandybridge then probably not. There are a couple of new architectural improvements, new RAND instructions and a new IGP but that's about it
i would only get ivy bridge if you are going to get a z77 mobo, other wise it would be fairly redundant IMO. i have a i5 3570k sitting on my shelf waiting for the full ROG ATX motherboards to be released.