If there is a chance that you might overclock, the 3570K might seem like something that you want, just in case. However, don't be discouraged by the cheaper i5s. They can still be overclocked by 20% to 35% or so through Turbo Core manipulation, so if you ever want to overclock, even those cheap Ivy i5s can get you a moderate overclock. Go for the 3450 or 3550.
EDIT: I had a typo where I said 435%, it should have said 35% and I've fixed it now.