I would ask around and buy based on who has a good service in
your area. You might also take some of the considerations mentioned
above into account too. I didn't see what boat you are powering, so I don't
know if the recent redesign of the Honda BF-90 is of interest to you
or not. Others might be lighter, better gear ratios, etc.
For instance, if I want quality Yamaha service, I have to tow about 20
miles, but only about 10 for Honda. Both are good shops. I don't
recall the state of Merc service.., but unless you live very close to me,
it doesn't matter. I don't recall any Suzuki service, but again, if there
was a good service shop relatively close, I would consider them too.
And don't forget the OMC engines...I am not going to make any claims
about weight, etc, you would be looking all that up yourself anyhow.
But, there are people happily running new OMCs on C-Dory's.
There's something to be said for looking at national networks if you
are going to tow a lot. OTOH, a well maintained motor will hopefully
not need a lot of on the road service.
There are some stories of interesting repair methods on this site. You
might want to avoid a brand, if possible, that puts you into one
of those shops.
I wouldn't worry too much about the idea of C-Dory dealer and 'one stop
shopping'. Once the boat is a few years old and out of warrantee, you
will not be taking it to a C-Dory dealer most likely unless you have a
dealer do virtually all of the work on the boat.
Mike