I'm surprised about your comment on the the 10" SCT getting a better view than the 10" dob. Usually SCT get a bad rap because of the large CO. I would have thought the views between the same apertures would at least be similar. Were both scopes at the same/similar power?
I'm in the southern hemisphere and M51 is too low on the horizon, same with M31. I've tried to see both. M31 just a smudge and M51 was too hard to find. Way down in the murk.
CO should have little or no impact on deep-sky observing. There are many factors that go into delivering contrast at the EP. In my experience CO is not that significant. The biggest is optical quality followed by mechanicals like baffling, flocking etc.