Two knobs
I have a knob on each side. Always have.I don't think I would choose to go to one knob. I find it useful to be able to balance the sound in each ear. Not everyone has the same hearing in each ear (especially with noise induced hearing loss as age advances) and it is the matter of a moment to adjust them when you have worked out where the knob are.
However as Jhieminga says I hardly ever touch them once they are set.
(I don't think it can technically be called stereo since we use only one sound channel split in two. I found out when I flew recently with a tank commander the army pipes radio 1 though the left hand headset and radio 2 through the right - that's stereo and very confusing it must be... If civil aviation ever goes over to this system you will need a stereo headset with two volumes. It also accounts for the fact that when you flip the switch to stereo one of the speakers cuts out, which has baffled me for 20 years.... )
I would stick with the more advanced product - but its your choice.