Re: Ivy Bridge vs. Sandy Bridge-E
it's difficult to give a definite answer as neither is released, but expectations are slightly different for each product.
E parts (sandy bridge-E and rumored ivy bridge-E) are geared for high end workstation. They have ludicrous amounts of memory bandwidth (4 channels vs 2 for the non-E), more PCIe lanes and generally "more" of everything (core, cache..), including of course price. All E parts are expected to be much more expensive than consumer parts.
To know which product is a good fit for you depends on what you are looking for, but E parts should not be great values, they are the best money can buy. See it a bit like 2500k vs 2600K, very few differences, especially if you overclock, but ~30% more in price...
As the universe is curved, there cannot be a straight answer...