I'm sure it'd definitely be much happier in the 18x18x24" then it would the 12x12x18". Not to mention, the price difference isn't that much.. if you're lucky you can find one for a good deal. I just bought an 18x18x24" from PetSmart a couple weeks ago for $75 brand new, nothing wrong with it. Which is a decent price from what I've seen advertised elsewhere.