Different motherboard / chipset architecture. The i7 9xx series has the memory controller on the processor die and supports up to 1066 MHz. The socket LGA1156 processors are designed to run with the P55 chipset and use the old FSB which supports the higher 1333 MHz memory bandwidth. A lot of debate out there right now over which is the better approach. Lots of review sites out there that have published comprehensive benchmarks and I'll leave you to draw your own conclusions, but any differences seen are generally pretty marginal. Personally I think the biggest advantage in going with the socket 1366 systems is having true x16 PCIe lanes but I recently put together an i5 system for a friend and was fairly impressed with the performance of what was a budget build. Unless you plan to move up to i9 whenever they appear, I think either option should serve you well.
