Both expressions mean the same but are to be used with discretion. Simply put, the the first expression is more polite ( can be used in formal conversations) and the second one is more colloquial (often used in informal conversations).

猫を買ったんです. when you say this, I expect you to continue to describe about the cat. which color, where you bought..how is she... which color... why you bought etc. ie: 'hey ! how have you been ?? listen! 猫を買ったんです. She looked like she was waiting for me... she is big and white...., and...' 猫を買いましたis like ie: "Hey, how are you doing? I just bought a lunch and .. oh by the way 猫を買いました yesterday. and I am thinking to buy a dog next week. how are you then..." difference is how detailed information you want to share. something like this... hope it helps. btw to go to asylem and meet cat is much better than buying a cat :)

I am interested where you find those two expressions.