On a stereo instrument or audio track, the panpot isn't a pan pot. It's a balance control that lowers the volume of one side of the stereo signal or the other. If you move the control to the right, the volume of the L-channel sound decreases while the strength of the R signal remains the same. And vice versa.
Example: you record a percussionist playing two shakers, one in each hand, and you're using two mics (a "LH mic" and a "RH mic"). As it turns out, when you're mixing, the L mic's sound is too loud with respect to the right. In this case, move the balance control to the right. This lowers the L-channel sound while maintaining the level of the sound in the R-channel.
The stereo panner is the same as the Direction Mixer plugin. Moving the control "rotates" the L & R signals in opposite directions in the stereo field.
Example: you have a sampled violin section patch that's recorded according to stage placement (predominantly straight signal in the L channel, predominantly room sound in the R channel). To center the signal in the stereo field, turn the control to the right. This moves the L channel towards the R while simultaneously moving the R channel towards the L, effectively "rotating" the stereo field and centering the straight signal.
The only downside of this approach is that the sound can tend to become quite mono if you move the control too far to one side, so be judicious with how you set it. Nevertheless, it saves the day when you need to change the centering of sample libraries that are annoyingly pre-panned.
On the flip side, you can use it to give more directionality to samples that are sampled centered in the stereo field.
So really, choosing balancing or stereo panning isn't a matter of one being better than another. The mode you choose depends entirely on what you're looking to achieve.