Well, they are pretty similar. The only difference is that the 2500k can overclock. And when I say overclock, I mean OVERCLOCK A LOT. Many people are hitting 4.5GHz on air, which is a HUGE increase in performance. However, you could probably hit 3.8-4.0 on stock cooling, which is still a big jump.
And when you consider how easy it is to overclock nowadays, I would go ahead and throw down the $35 to get the 6870+2500k. Just my opinion though.