SB: Uses Flux-less solder to mount heat spreader to chip.
IB: Uses Thermal Paste, less efficient cooling. hotter temps.
SB: Draws more power.
IB: Much more energy-efficient.
Ivy Bridge introduced a new technology of making 3D transistors that save tons of space, so they cost less for the power, draw less power, and produce less heat.
I'm not sure what Intel was thinking, though, about using thermal paste on the heat spreader.
I've got a question for those out there, though: I'm looking to upgrade. Should, I get, say, a i5-2400 (SB) or an i5-3550 (IB). I want to utilize my full ddr3 1600, but I would like something with, you know, fairly good cooling efficiency, too.