I don't know what material the new steel ones are made of, the old one was supposedly sintered steel that had propensity for cracking on over reved or high mileage engines. I have seen the titanium material recommended several times on this forum. Besides being an alloy and not having the propensity to crack as the originals, it is also significantly lighter.
Since these would weigh so little, it might be worth to get a quote from Steve Weiner at Rennsport systems:
Welcome to Rennsport Systems, Porsche Performance Products for the 21st Century
He is very competitive in price because he knows little tricks like, use 993tt heads studs with 964 nuts so you don't have to buy separate washers, they are built into the 964 units. He is very knowledgeable.
I imagine you will probably have your heads rebuilt in the UK, but if you are considering someone here in the US, in no particular order and based on comments I have seen here (not personal experience):
1. again Rennsport systems
2. William Knight :
Knightrace Performance-About and Contact Us
3. member on this forum by the user name: cgarr