In general, growlithe tend to be among the easiest pokémon to train for domestic purposes, hence why they’re far more popular as pets than houndour. Don’t get me wrong, of course. Houndour are also excellent pets and far better guard pokémon. It’s just that they’re less friendly, more stoic, and otherwise pack creatures. They’re not the sort who will be eager to entertain the way growlithe are, and they fare much better when homed with at least one more of their own species. As such, it may be harder to get them to listen to commands than it would be for the notoriously loyal growlithe, which means it may be harder to housebreak them, walk them, and generally train them.
On the other hand, they’re also less excitable than growlithe, and thus, they’re less likely to bark or use their fire abilities excessively. Very minimal fireproofing is necessary for a houndour, whereas a growlithe may howl, pounce, and use Ember in celebration for your return home after a long day at work. Thus, for a houndour, you may only need a handy, charged fire extinguisher in as many rooms as humanly possible, whereas growlithe need training and a constant water-type partner at minimum.
In short, I supposed it would depend on your needs. Growlithe are easier to train and more affectionate, but houndour are quiet and well-behaved by default but not quite as emotionally available as growlithe may be.
One last note, though: if you have small children or are expecting, contrary to popular belief, houndour are excellent choices—far better than growlithe. Not only is this because houndour are generally calmer, but it’s also because their pack instincts kick in around human babies and toddlers. The truth is, you’ll likely find no canine pokémon more protective and concerned for a child’s well-being than a houndour, even though the species is often (wrongly) associated with violence.
Hopefully, this helps, anonymous. Best of luck with your decision.
The Pokédex According to Bill 