The difference in this case is that Pan Knob won’t sum the low frequencies to mono. It leaves them untouched. So if you have a stereo track and put pan knob on there without panning, it shouldn’t make any difference to the sound, no matter where the crossover is.
Mongoose, on the other hand, will sum the low frequencies to mono.
The CPU usage for pan knob should be a little lighter than Mongoose, although both should be really light. I put 30 instances of Pan Knob on a track and it used 0.2% cpu on my machine.