Depends on what your looking into do with the engine. This isn't something you drop in for a standard replacement of what's under the hood, your spending $$ to have the block machined and bored to your specs most likely for a racing application.
The aluminum block saves weight over cast iron, if shaves a 0.1 of a second off your time that could be the difference between winning and loosing. The less weight also can contribute to a better weight balance of the vehicle (front to rear), again your paying for the advantage.
Aluminum will also reject heat better than cast iron. The cast iron will tolerate overheating better than aluminum, that probably the biggest driving factor why it's used in heavy trucks.