Why isn't there an option "I like both"? Because I really do.
I like cats because they're not as much of a responsibility. You can even leave the house for 3 days without having to get someone to look after them. They're lighter and easier to handle and cuddle with. You don't have to bathe them as often, you don't have to look after them as much, and they're just a lot less needy in general.
I like dogs because you can do more stuff with them. Take them for walks, take them for car rides, take them fishing with you, play more games with them, and the list goes on. I wouldn't ever own a small dog though. If I was going to get one of those I'd just get a cat. But middle-sized and even larger dogs feel more like an actual family member than a cat does.