The Newcastle core was made on the 130nm process, whereas the newer Winchester core is made on a 90nm process. Winchester cores run cooler, overclock better, and hence are the core of choice.
Winchesters will run cooler, and may O/C better, though that's certainly not a given. If you are intrested in O/C'ng, then you may want to wait for Venice core.....should be avaiable in the next month or two.....
That's not quite true. The 90nm chips ARE faster, though not a lot. 1% or so maybe overall, just because some tweaks were made to the chip design (a venice is very slightly faster than a winchester, which itself is very slightly faster than a newcastle).