__declspec is a visual studio keyword that CVI also supports. __declspec(dllexport) is a separate keyword from the calling convention. You can use both with the same function. TestStand supports both cdecl and stdcall so either is fine if you are only using your dll with teststand. If you are using your dll with VB6 for example though, you must use stdcall. So it really depends on how you are using your dll.
-Doug