The i7 would have to very cheap for it be an attractive choice, as keep in mind it demands a more expensive X79 motherboard and is based on pretty old architecture (it's a Sandy Bridge-E chip). Throw in the fact the X79 chipset had no native USB 3.0 ports, and only 2 SATA 3.0 ports, and you begin to see why a more modern i5 (or i7), with an appropriate Z97 board, is a more attractive option.
Really, the only reason to go for the i7 3820 is if it's very cheap, or you have particular need for the 40 PCIe lanes it has.