Both lumps are good and I have heard that the Duratec is awsome when it comes to N/A tuning, but isn't well suited to turboing as reliabilaty is a problem. But it is currently the favorite for amature rallys. See a few Mk1's and Mk2 with Duratecs in them and they are very competative.

I dont know from experience but someone said to me that the Duratec has totally different mouts so it isn't a straight swap. But then again you can always just get the V6 one and it wont matter to much.

Duratec V6 Mk2?

As said, Zetec is heavy (every tried lifting one of the bastards ) and the head isn't as good. But the bottom end is solid up to 400bhp and requires little work to get to 250-300bhp.