I look for affordability, durability, does it meet the animal's requirements, how easy to clean/maintain, and does it look nice. This is pretty much why I've built all of my enclosures. Some of the ones on the market are very nice; but I can build one that works just as well for a fraction of the cost; that's my main reason for building over buying. But if I was ever to buy one, I'd still use the same criteria.