I believe this is only true of heating/ cooling cycles. If you are "tootin" down the road at a fairly steady pace, both manifolds would probably be about the same. But many guys mistakenly think "aluminum runs cooler" because it's associated with stuff like heat sinks which RADIATE heat. In this case, as on a V8, an aluminum manifold has most of it's area heated. IE the heat conducting into it from the heads, the (after warming) hot oil underneath, and the exhaust crossover.
The cooling effect I mentioned comes from the low pressure and evaporation of fuel, which can be considerable.
