Not sure I understand the question. Totally different settings.
Combine makes a single mono track out of the two channels of a stereo track. End result is mono.
Both Channels is the default where you have both the right/left of stereo, or 1/2 of dual mono. End result is 2 channel/stereo
For voice over, for example, it seems that there would be no difference between using Both setting or Combine setting, if there were no effective difference between right and left stereo channels during recording. Correct?