Sorry. You should always go with what sounds best to you. If you only hear in one ear, then the aural illusion created by stereo is lost completely. Like there are optical illusions that tend to fool the eyes/brain, stereo is an aural illusion that allows placement and location movement within the environment that can play tricks on the ears/brain. A big part of motion in music recording is this aural illusion. And the placement of instruments in the stereo panorama.

As BM has written, there are many reasons to go for Montage over the XF since it is the latest technology. The removal of the sequencer may take a while to adjust to if you have never played with DAWs before. I admit I use the XF sequencer less and less all the time, although I still use it for sketching out things. The Montage still allows this but without editing features. With either the XF or Montage transferring the sketches into a DAW is relatively simple.

That said, as someone who did the ES to XF move I can tell you the amount of work you will need to do is not great. Sequences and performances do not transfer directly, so there is some work to be done there...it really depends on how much material you have to move. The voices and samples transfer directly with no work needed, I still have Motif Classic voices in use on my XF!

Although I am not selling my XF to buy Montage, if I was in your situation I would definitely buy Montage and move to a DAW for sequencing. Assuming Yamaha still provides Cubase AI, you will be ready to go out of the box.