I've heard that there is not too much difference between them except for multithreaded applications. However, how much is there for the multithreaded ones?
What are you doing with this CPU? If you're playing games you would be better off with the 2500k. With the I7 you're paying an extra 100 dollars for hyper threading and extra cache, two thing that games really don't make use of right now. If you're doing something more CPU intensive like video editing or 3D graphic design you would be better off with the I7.