I own a Blackstone now, and a cast iron griddle, and a Magnalite Pro alloy griddle and a lightweight aluminum non-stick griddle. All have their place and each has it's own advantages and disadvantages, yes, even the lightweight aluminum one.
I do not fully agree that cast iron is always the best option for griddle work, note that Benihana could easily use cast iron, but, they prefer steel. If you cannot justify owning one of each, than a cast iron griddle on a gasser is a great way to go. But, the Blackstone is incredibly well built, the heat controls on the 36" I have are vastly superior to any gasser in the same price range and the steel griddle is very heavy duty.