I'm pretty sure that Ophelia could understand quantuum physics if it were translated into Meow. She is VERY smart! I've seen her using logic to figure out how to get at a toy that she lost (it was a mouse on a door string and she pulled too hard and it ended up on top of the bookcase headboard).
We had a dog that was very smart too. Like a cat, she didn't let herself be trained, either. She could have, she just didn't want to learn "come" and "heel". But she learned any trick if it involved treats. I think Felicia was a cat at heart.
I do think it depends on the animal. There are some very smart dogs and some very dumb cats. Being able to be trained doesn't determine intelligence, though, as many have pointed out. Labs are notoriously dumb dogs, but they can be trained well.
I voted for cats.