Ivy Bridge will be socket 1155, and will work in some Sandy Bridge boards (H61, H67, P67 and Z68 with an updated BIOS).
X79 boards are socket 2011, so not compatible with Ivy Bridge. There will be Ivy Bridge-E chips in a year that will be sockcet 2011 and will work in an X79 board.
Which is best for you depends on what you will be using it for. If you are mainly a gamer, I would recommend Ivy Bridge when it comes out. Specifically the i5-3570K, which will replace the 2500K. If you do a lot of video rendering or other multi-threaded work that will benfit from more cores and hyperthreading, then X79 is the way to go. I believe the i7-3820 is due next month, and that is a quad-core (but with locked multiplier) at under $300. It would offer an upgrade path to 6 or 8 core chips in the future.