i have had many hegels and also have a stratos stereo amp with the various upgrades
i think the big hegels will have an ever so slightly more detailed presentation (i am talking about the amp section only of course) and a smidge tighter bass, whereas the odyssey is a little smoother a little more laid back up top and have a little more bloom in the bass... both will have very nice midrange and excellent top to bottom coherence
i really don't think mono vs stereo matters much here, it is about the circuitry and the quality of design and build of the amps here