Both are incredibly cute. Definitely in the top cutest of Unova alongside Deerling, Munna, Oshawott, Duckette, Sewaddle, and Petilil.
Lillipup's face looks like he is saying "do you love me?"
Purrloin's face look like she is saying "dontcha just love me?"
The decision is tough, but I will go with Purrloin mainly because Liepard is in my third favorite Unova pokemon. Liepard obviously is not anything to brag about in terms of battling, but I love her design!!! Moreover, in-game she is actually useful to cleanup with fake out/sucker punch + high speed stat, has prankster ability + thunder wave, and can eject herself from battle with u-turn to get the hell out of there after clean up or paralyzing the opponent.
So, although Purrloin and Lillipup are equal in my eyes; Liepard's design and usefulness outshines the Lillipup line.