Well I don't really think it's even a question.
With sandy bridge you are getting higher IPCs, as well as referring to an i5 model, you are getting twice as many cores. E.g The i5-2500k vs the i5 655k
(Correct me if I'm wrong) the i5 2500k is a quad core variant while the i5 655k is a dual core I think.
Also you're getting better oc potential, and faster single core speeds due to the lack of Two physical cores.
Also, since it is a newer variant, it probably won't be as much of a bottleneck compared to the latter
If not convinced, or you want details, check out websites like these:
http://www.anandtech.com/show/6808/westmereep-to-sandy-bridgeep-the-scientist-potential-upgrade/6
Or just put in the processors in cpuboss to get a rough idea of synthetic benchmarks