Which is correct: "dogs are animals and they are very helpful" and "although dogs are animals, they are very helpful" ??

Dogs are animals, and they are very helpful.是「對等句」，對等連接詞and前後兩個子句裡的資訊同樣重要。
Although dogs are animals, they are very helpful.是「複雜句」（complex sentence），在「主要子句」裡的資訊比「從屬子句」(although 是從屬連接詞，它所帶出來的是附屬子句) 裡的重要。