For all intensive purposes, they are the same. It depends on what you want do do, how much HP, stroke, deck ht, etc. Dart and World Products (makers of Merlin products) used to be owned by the same people. World Products was the part of Dart that was responsible for most of their cast iron products with Bill Mitchell in the design corner. Dart was more focused on the hard core race products, including aluminum heads and manifolds. Dart also casts a lot of aluminum parts for other companies. A while back, the two companies seperated, and are now independant of eachother. It's still my understanding that the Dart products are more directed at hard core racing, but I don't know how different the two blocks are, except that the Dart is probably available in more configurations than the Merlin. Someone else might know better. You could call the two companies and ask.