You are correct in that they are SB based because IB has not been released yet. They are still pretty close between though since you are considering the current line of IB in other socket formats. You shouldn't compare the 2011 socket SB via over other IB sockets though and think about when you want to go IB in a few years with the 2011 socket format.
Just check out the bench marking and what 2011 brings considering it is only SB and the particular one is 4 core.
http://www.anandtech.com/bench/Product/551?vs=523
Also it supports more PCI-E slots which run at x16/x16 rather than x8/x8. It also has Quad Channel Memory, 8 DIMMS. You can run up to 64GB ram. It is future proof for new drivers, better ability to run multi GPU's etc so much more flexible.
The reason for 4 cores in the cheaper chip is because it is actually an 8 core that some cores didn't clock so they disable them and run it as 4 core to save money. You wont notice the difference for 99% of what you are going to do and can easily upgrade to IB at a later date which will bring even more benefits without needing yet another new mobo.
Kind Regards,
Adam
Edit: It was different when they first came out also as 2011 socket cost more but now they are pretty much the same price. Bare in mind this is the latest tech. The 1155 sockets are at their final IB will be the best and last version so if SB versions of 2011 are running at the same if not better abilities already it can only get better.