The Mackenzie is a big river and it flows fast all the time. You would
be able to cover a lot of distance without paddling. The north end is
interesting as I understand it - lots of shallows and stuff.

It's big AND boring. A big lake with a lot of current.

The Horton, OTOH, is in the middle of nowhere.

Did the Horton last summer, Horton Lake to the ocean in 21 days (it's
about 600 km). It's a Barrens trip, you gotta like solitude and flat
land. It's more about the journey than the scenery. A sat phone is a
must if you're worried about a quick rescue. Temps from 33C to 0C. Some
class 3/4 whitewater in the middle for a couple of days, otherwise easy
class 1 (assuming you're comfortable in moving water). We flew in from
Normal Wells on a float, out to Inuvik from the mouth on a Twin Otter
(wheels). Saw a few other people along the way. It's a lot easier with
an outfitter; arranging those 1-way flights and renting boats gets ugly.

(I'm only lurking here for a while, killing some time, after a long
absence; email if you want more info).