The performance is virtually the same -- the 860 scores 5527, the 920 scores 5576 on PassMark's CPUMark.
The 860 supports the newer VT-d virtualization feature, which provides better support for I/O devices in virtual machines. (and is required for some newer hypervisors)
The 920 is a Socket 1366 CPU, which has a better upgrade path to higher-performance hex-core CPUs; supports more PCIe lanes than the 1156 boards; etc.
Which is "better" depends on the overall system you're planning to build -- if you need the higher performance characteristics of an X58-based board, you need the Socket 1366 920. If not, I'd go with the 860 => primarily for two reasons: (a) the lower power consumption (82w vs. 130w TDP); and (b) the VT-d support.
Details taken from:
http://www.cpubenchmark.net/cpu_list.phphttp://ark.intel.com/Product.aspx?id=41318&processor=i7-860S&spec-codes=SLBLGhttp://ark.intel.com/Product.aspx?id=37147&processor=i7-920&spec-codes=SLBCH,SLBEJ... and my knowledge of the different buses & chipsets
