870 (code named lynnfield) and 920 (code named Bloomfield) are of different breed .

870 is more recent architecture, albet with few shortcomings . If you want latest virtualization features go for Lynnfield (vT-x & vT-d IO). But will be limited to 2.5GT/s (21.3 GB/s) bandwidth and dual channel RAM bandwidth(upto 1600 Mhz). PCI-e of 2 (8x8 ). 870 has 2.93MHz .

Where as 920 is old horse from Intel's stable , it has 4.5GT/s (~25.6 GB/s) and supports tripple channel RAM (limited to 1066 MHZ).PCI-e of 16x16. 920 has 2.66Mhz. 920 is more open to overclocking.

So both of them has pros and cons ... what you choose will depend on whats ur choice about bandwidth and CPU frequency.

Reference : http://tech.icrontic.com/articles/making-sense-of-lynnfield-is-bloomfield-really-better/