Growlithe. He matches my battle strategy better, is easier to raise, is cuter, has a much cooler evo, isn't overused by the grunts in his respective game (and many grunts in games afterward), has better stats, and was one of the original 150. So, I'm gonna have to say Growlithe. (Though, I'm biased since he's one of my favorite Pokemon and always has been.)
I tried raising a Houndour/Houndoom once and I just... totally failed. I ended up switching him out for a pure fire type. There's something about Dark type Pokemon that I just don't mix with. (Perhaps that they're more... "sneaky" based, per se, than outright attack based?) Houndour is cute, though. Have to give it credit there.