Dogs are typically way more dependent on social interaction, all-day abandonment of a dog would ammount to animal cruelty imho. But for either of them 12 hours is pretty harsh, even for a cat.
Remember, a dog will have to hold in any "bathroom duties" untill you come home, and 12 hours is pretty rough. It also is unable to really portion its food, depending on you to feed it specifically. I know some people who have to walk it quickly before work, for example, to empty the bladder. Either way, it's a lonely life for a dog
With a litter box, and some toys, a cat *could* deal with it. Cats also portion their meals on their own, so you fill a bowl with food, and a cat will eat what it feels like, when it feels like it (while a dog will eat *everything immediately*), but yeah, if you have the option to wait for the oportunity for more spare time, then that's much more ideal
Yeah, and there's also the whole thing where you'd get most pets as infants, as in, puppy or kitten, and they require a *lot* of attention. Alternatively would be a resque-pet from a shelter, allready trained to some degree. People getting kittens or puppies tend to time this with long vacations, for that reason