While iron engine blocks are time-tested and race-proven, aluminium is the preferred metal for many racers. It does have many advantages, but you need to be aware of its limitations.
The primary argument against aluminium blocks is the cost, which can be more than triple that of a comparable iron offering.
Aluminum's primary advantage is its light weight. Many blocks are half the weight of traditional iron blocks. This is especially important when you consider that every bit of that weight is over the front axle, which is a detriment to handling.
While a properly designed and prepared aluminium block can be just as strong as iron, iron is inherently stronger and more dependable in high horsepower applications.
Aluminum's tendency to distort under stress can damage the cylinder bores and widen clearances, both of which are a detriment to horsepower.
Because they expand at the same rate, aluminium blocks are less likely to experience blown head gaskets when used with aluminium heads than iron block/aluminium head combinations.