The prices could be very similar and so could the performance. The Ivy Bridge CPU's should be very efficient, you can bet on that. The issue is there isn't a review of the product yet and it's not on the market so it's difficult to say that the 3570k will be worth the money. My best suggestion: if you're curious enough to ask and not in an immediate rush to buy a new CPU, it cannot hurt to see what Ivy Bridge can do.
I have, and love, an i5-2500k. It's a fantastic CPU and may even see a price cut (I'm speculating) when Ivy Bridge comes out. So at the very least, you wait a month and might get a better deal out of it if you do decide on the 2500k. On the flip side, if the 3570k is a much better CPU and costs around the same, then you made the right decision to wait. It could be a win-win, but I'll leave that up to you!