Neither. But she is not a mug ( well, maybe sometimes, :lol
Li's serve used to be very bad. She has improved significantly over the past 2 years, but still it can break down badly.
Her serve can be a weapon sometimes though. Na's RoS is okay as best. However her weapons are her movements, blistering groundstrokes and the ability to hit winners. Just like Caro's weapons are movements and consistent groundstrokes.
Li's biggest strength is her ability to return her Opponents pace back with added interest (kind of like a super counterpuncher for lack of a better term). With that being said her ROS should be the better of the two, considering she has to produce all of her own pace on the serve whereas on the return she can redirect the pace of the server.