Ferrea or Manley valves -which is better? This is for a turbo motor so the exhaust valve has to be very good quality.

They both have different levels of quality. The best in either line are comparable. theres always titanium$$$$

I run the Manley Severe Duty's on my turbo'd 383 now. It's really just an insurance policy for me though. I've had the regular old street flo valves that my heads came with on this thing for like 4 years. Two years with a blower and two with the turbos (up to 14 psi) and there was nothing wrong with them when I changed them. Only get the Inconel exhaust valves if you are wanting to run under continuous heavy loads as a boat would be or really going to go crazy with the boost.