My instinct is that you should drive via Montreal, Riviere-du-Loup, and Fredericton.

There really isn't a majorly useful path through the states that would make it worth stopping at two border crossings on the way. The distance is similar, and perhaps even more if you travel through the U.S. Don't know if you can even save much on gas (via the U.S.), given that so much of your trip needs to be in Canada anyway.

The problem is, that to drive to NS, you effectively must go through New Brunswick, and that's way up high there, so that renders a trip down to Boston somewhat out-of-the-way.

If you enter at Calais border crossing, the road bogs down with a slow passage.

The drive itself should be rather easy and harmless (save for winter storm effects) and the main highways between Mtl and Riviere-du-Loup figure to expedite the process as much as you can hope.

For what it's worth, I've done a straight through drive from Halifax to Montreal with three drivers spelling each other off, which was doable but somewhat arduous.

Given the added distance you're looking at, and having only two drivers, I'd be somewhat concerned about driver fatigue. With three drivers, one could be sleeping, one driving and one keeping the driver entertained and alert.

It's really way too tiring, especially this time of year, please stop overnight for your own safety and the others on the road.

Its about 17 hours if there is no traffic or weather problems, driving through Canada is far simpler, all major highways, to go thru the US you have to take many minor roads and deal with border crossings, probably way more time.
You really need to stay overnight somewhere, Quebec city area would be about right.

I estimate 2000 km, based on my home in Toronto to my grandparent'shouse in Moncton at 1000 miles.

The roads through Canada are, as noted already, pretty much all superhighways, or at least good wide paved straight as possible roads.

But it is winter in Canada, and there's cold weather and a lot of snow is highly likely for a lot of the trip.

And there's long stretches of forests and farms with no towns and no gas stations and no nuthin' of any kind of hospitality and safety-shelter.

On the other hand, going through the USA is not much longer, or no longer, depending on what route you take. But any route involves some twisty, turny, country and forest roads, and even if you go out of your way to stay on bigger roads like the New York Thruway and the Mass Turnpike, remember the words from a James Taylor song about the Turnpike being covered withsnow from Stockbridge to Boston.

My point being, it's a long drive through somewhat dangerous country. Perfectly safe if you watch the weather and wait for a storm to pass, but it could be awful if you hit an ice storm in Montreal that turns to snow by Quebec City and then sleet by the time you get to Fredericton, and snow again from Sussex to Truro.

Build in some extra time, and some extra budget.

You can stay in Edmundston, NB (about 1/2 way on the ON/PQ/NB/NS route) for less than $90 per night at this time of year-you would be VERY wise to consider this option with only 2 drivers.

DH and I lived in NS while I was still working in TO for 5 years so we did this trek, winter and summer more than 30 times. You are really asking for problems trying to do that drive at this time of year without a good night's sleep.

The US route is slightly longer...maybe 19-20 hours versus 17-18.