Welp, I've never driven through the Maine part of that... BUT my experience along the St. Lawrence, via Riviere-du-Loup, was so pleasant that I would not go through Maine if offered the chance to go from Mtl/QC to Nova Scotia again.

Not only that, but avoiding a couple of border encounters seems sensible in the summer too.

Were it me, I'd go Mtl... QC... Riviere-du-Loup... Fredericton... Sussex... Alma... HOPEWELL CAPE... Moncton... Truro... Antigonish... Baddeck.

Whatever you do, go to the following link and print out the TIDE TABLES for the Bay of Fundy at 3 or 4 locations, during the ENTIRE window of your trip.

http://www.lau.chs-shc.gc.ca/cgi-bin/tide-shc.cgi?queryType=showZone&language=english&region=5&zone=30

Do it NOW... and then put it in the glove box for constant reference while touring in NS.

Include Hopewell Cape... Truro... and maybe one or two others.

IF you go to Hopewell Cape on the way IN... you'll have covered it... (at LOW tide, hopefully)... and then you can pass by at high tide on the way OUT of NS... for the needed contrast.

I tried Google maps to see what they said as I was interested myself - the only time we drove through Maine to get to Atlantic Canada was many years ago and it seemed to take a long long time.

However, if you put in Calais Maine as an intermediate destination (between Montreal and Baddeck), it gives a route which takes, according to them, just about the same amount of time as the route going through Canada (Riviere-du-Loup then through New Brunswick). This is "not" the long route we took (we stayed closer to the Maine shore and overnighted near Bar Harbour). But NorthwestMale brings up a good point about border crossings which can be busy with line-ups.