Board Thread:Questions and Answers/@comment-112.205.202.205-20150224202018/@comment-24659969-20150226023652

Literally, there is only one rare that is good to exchange to XP, which is the golden cat for. If you haven't unlocked the cat, it would be better to do so rather exchange one to XP. Especially for supers and ubers since there is no guaratee that you could obtain same one in short future.

As for your 2nd question

Whale cat is easier to level up than Viking cat. Also if you could maxed whale cat, it simply is stronger than maxed potent of Viking cat