Since most of the energy in musical peaks lies in the bass region for typical material, you will have more usable power from the 120W 2-channel amplifier. Although keep in mind that the difference between 120W and 60W is only 3db, significant but not huge.
Also, everything else being equal, I would think that there is more opportunity for inter-channel (left-to-right or right-to-left) crosstalk or interaction across the power supply and/or grounds within the 4-channel amplifier. That may or may not be significant depending on the quality of the amp.
But as with any component selection, of course, generalities can often be over-shadowed by the quality and sonic character of the specific designs, which can best be assessed by listening.
Regards,
-- Al
Also, everything else being equal, I would think that there is more opportunity for inter-channel (left-to-right or right-to-left) crosstalk or interaction across the power supply and/or grounds within the 4-channel amplifier. That may or may not be significant depending on the quality of the amp.
But as with any component selection, of course, generalities can often be over-shadowed by the quality and sonic character of the specific designs, which can best be assessed by listening.
Regards,
-- Al