I like both, currently only have a cat at my apartment and a dog that stays with my parents til I get a place with a yard.
My cats a pretty cool cat though, more dog-like than most. He'll fetch, chase ping pong balls etc. Lay with you at night, stand on his hind legs begging for a treat, come running to you when you open the door, is fine around dogs (unless it's one that doesn't like cats or that he doesn't know very well), he can do commands like sit.
Also, which is probably the best thing. He's very laid back and loving, and he doesn't use his claws (though he still has them). You can lay him on his back and rub and stomach and he enjoys it. Or on the couch you can sit him on his butt like a person and he'll sit there for 5-10 minutes.
I've never had a bad inside cat (only had one), but having bad cat or dog inside would ruin it. Eventually, I'll get a little puppy for inside too. Probably gonna go with a mastiff