Love On Top is harder to sing, at the end she goes up and Up and UP! That is not an easy feature!
All you need to have to be able to sing IWALY is a nice belting and breathing technique just on the chorus. There are plenty successful youtube covers of IWALY and many different ways to sing it a la Dolly or Jennifer Hudson. Whitney just made it her own, but Love On Top is the harder song to sing starting from scratch.
Beyonces difficulty lies in the details, for instance the "Come on baby, it's youUuUuUu" is nothing something most can do.