IB is a better choice. smaller die. runs cooler. less power consumption and plus from nehalem and sandy bridge you could tell that IB will be better than SB-E.(like 2600k is on par with 980X)
and of course nothing's better than having the latest gen
your system is still awesome. it may be better to skip sandy bridge and go straight to ivy.
and of course wait for matured benchmarks to come before buying either..
If I'm in your place I would go straight to Ivy Bridge.