Off course, site selection is always more important than species selection but in some cases these take each others' place, alternative ways! For example, If you choose a saline place for freshwater fish or any other animal's culture then your input will be so high than output.
They are equally important. Choosing the correct species for a particular set of environmental conditions is a recipe for success. A good species in the wrong environment is a recipe for failure.
Yes site selection is important because it ensures basic requirements for culture operations, avoid pollution threats and other social issues arise in future, which are required for successful culture of any species. Species selection is depend upon the existing environmental conditions, demand for the selected species in local and international markets that lead to more profits.