I would match the opposing height under the overhead cabinets. Symmetry and consistency is more appealing to the eye naturally.

Your installer is correct. Same height on both side of the window, .................what are you saving 8 tiles?

Always bring it to the bottom side of the cabinets bare minimum. The glass tiles are specially made to fit your standard 18" backsplash depth. What color glass tile are you going with?

Although it may be considered outside the box, I would put the backsplash just under the upper cabinets. I think continuing the 22" backsplash on the left of the window will further highlight the asymmetry that there is no upper cabinet.

Thanks for your thoughts. I think I will go with the 22" next to window. It needs some backsplash being so close to the sink and by coffee maker. It is a tannish colors (similar to walls) with irridescent, matte, and clear tiles.