What makes Cas so special to the Winchesters in comparison to Adam?
How is this even a question? Castiel has done so, so much for the Winchesters and yes, quite a bit of what he’s done for them goes unnoticed or under-appreciated, but PROFOUND BOND. Yeah, Adam’s their brother but it’s like how Dean was with Samuel and his family; he recognized them as biological relatives but not as blood, as family. He and Sam didn’t really know Adam and yes, they don’t know Cas all that well either, but their closer. You know, profound bonds and what not.