I am building a set of 496HO's and my cores came with an Aluminun set and a 2 sets of Iron.
my thoughts are to buy another set of aluminum and sell the iron ones. what are the benifits and disadvantages of either one
my boat is in fresh water and they are both late model units with heat exchangers and coolfuel boxes with the plastic fittings

Aluminum usually have better flow with less restriction and lighter then iron. If engine starts to overheat they can be damaged easier. Iron in most cases are trouble free for fresh water boating and can take the heat. I pulled iron off and installed aluminum with stainless elbows.

Weight is the only difference between the aluminum vs. cast iron 496 manifolds. Both used stainless risers.

about 40 lbs each

I like the weight savings..what about heat disapation.

Fast, I know a lot of the other folks will shell out 4 or 5 grand for exhaust but to be honest the 496 Merc aluminum exhaust manifolds are pretty damn good. No issues with heat, dry joint riser and manifold, divided exhaust runners. I use them on all my motors.