In both series a human boy lives in this big mansion with a number of monster girls for some reason. Oh, did I mention there are the monster girls in both series?
Anyway, Kaibutsu Oujo has cooler characters and (probably) better action scenes, but Ashita no Familia doesn't come all that far behind when it comes to being entertaining.
Staz isn’t what you’d call a traditional vampire. He’s fascinated by human culture - especially anime, manga and video games - and unlike his brethren, he has no interest in sucking people dry. As a prominent street boss in the demon world, Staz has never desired to go to to the human world. But when a human girl accidentally wanders into his territory, is eaten by a carnivorous plant and comes back as a ghost, Staz makes a life-changing decision: he’ll help resurrect this lovely lady any way he can, even if it means heading to Earth! But the path won’t be easy, what with having no idea how to complete the task and his own laziness standing in the way.
Kaibutsu Oujo and Blood Lad have very different style, but very similar themes. In both a human being gets mixed up with monsters and dies. But things dont' stop there for that person. From now on he\she has to continue living(?) with the denizens of the monster world. Not to mention that vast majority of characters in both manga are monsters themselves. So, if you liked one of those two, you should give the other one a try too.
If you enjoy seeing a lot of references to classic action (and non-action) movies in your manga then Boku ni Koi suru Mechanical and Kaibutsu Oujo might be series for you. Both do this in spades, though Kaibutsu Oujo also has a lot of game references.
Edit: Fixed the manga name. Was suppose to be Kaibutsu Oujo instead of Kaibutsu Ouji...
Both Kaibutau Oujo and Ga-rei can be episodic at times and when that happens they get the monster-of-a-week feel to them (though there are still ties to the main storyline in both cases). The enemies in both of those manga are usually taken from lore and urban legends. The style is different in those two, but if you liked that rich collection of various monsters in one of those manga, the other one might interest you as well.
Yuuhi Amamiya is a loner whose mistrust of people was instilled in him at a young age by his abusive grandfather. So when an annoying talking lizard tells the university student that he is one of twelve knights chosen to stop the gigantic "biscuit hammer" before it smashes the Earth to smithereens, Yuuhi has no inclination to rescue such an unkind world. Despite his apathy, the boy is targeted by a golem while going about his day-to-day life, and ends up teaming up with the other knights anyway. One of his new companions is Samidare Asahina, the "princess" and leader of the knights, who secretly is only trying to protect the Earth so she can destroy it herself! Entranced, Yuuhi swears eternal loyalty to this cheerful devil that promises to crush the world he despises.
Both starts with a Princess getting a servant to help them which later on develops into something like soft romance. The servant obtains super special cool powers so I recommend it because its about the same art and character artworks. If you enjoyed any of these I guarantee you will enjoy the latter.