B75 offers "native" PCI support with Ivy/Sandy Bridge, which may be relevant if using older PCI cards. It's a "business-oriented" desktop chipset, so not intended for overclocking/gaming, although several mobo makers do offer B75 boards with overclocking support (and ridiculously, dual PCIe-16 slots).
The second PCIe x16 slot simply aggregates four of the eight available PCIe lanes from the PCH. The (mechanical) x16 form factor allows any PCIe (x1, x4, x8 or x16) card to be installed, though obviously the performance of any such card will be limited to PCIe 2.0 x4 speed.
Also, onboard RAID support is iffy.
That's an interesting statement. Care to elaborate?
3820 requires an X79 mobo, and only makes sense if one needs more memory or graphics performance.
Or expansion capability (up to 40 PCIe lanes from the CPU, as compared to 16 from Sandy/Ivy). Or - if the DAWbench results posted by ADK are any guide - sheer muscle.
Oh, and you won't find HDMI/Displayport on B75.