[shadetree]

IMO, aluminum heats up faster (needs less "choke time" in cold weather), are lighter, and are usually improved in the breathing department over the cast iron units. On the minus side, aluminum can be harder to seal (aluminum expands much more than cast iron, making gaskets harder to get to seal). Aluminum manifolds usually cost more.
For your build, either material should be OK, with aluminum getting the nod if you can find a good one, cheap, and you remember to use the reccomended gaskets for it.