"Urusei Yatsura is the story of the unluckiest and most lecherous young man alive, Ataru Moroboshi. When aliens decide to invade Earth, Ataru is randomly selected to defend his planet by playing a game of the aliens national sport, tag. Should he win, the world shall be saved. However, Ataru is motivated for far less noble reasons, as the one he plays against is revealed to be the curvaceous alien princess, Lum. The game goes for 10 days, and on the last day, Ataru, motivated by his girlfriend assuring him marriage (and consequently, a marriage night one can assume) should he win, finally catches Lum. However, with his victory he gives a cry of joy for his coming marriage, which is misinterpreted by Lum to be a marriage proposal. She promptly agrees, beginning the two's "marriage" together."
"The story is about Rito Yūki, a high-school boy who cannot confess to the girl of his dreams, Haruna Sairenji. One day when coming home and sulking in the bath-tub a mysterious, nude girl, appears out of nowhere. Her name is Lala and she comes from the planet Deviluke, where she is the heir to the throne. Her father wants her to return to her home planet so she can marry one of the husband candidates, but she decides that she wants to marry Rito in order to stay on Earth."
The biggest differences are:
wimpy, non-lecherous hero, and less overall comedy at the expensive of giant boob fanservice shots every page.