The Trench route would really only be a good idea IF the weather was bad on the highway route. This is due to the Trench route going pretty much due south, so longer if going east.
Also, from Watson Lake, I would not necessarily follow the highway the whole distance to Ft Nelson, I'd cut north of the highway route, as it enters the Continental Divide, and follow the lower terrain to the north. Lots of gravel bars there to land on. Lots of BIG rocks along the highway route.
I've cut straight across from Watson Lake direct to Fort Saint John on a stone clear day, but there are very few nice places to land if anything goes wrong.
Once at Ft Saint John, I'd angle as far east as possible, and work my way through the Canadian prairies, as opposed to dropping further south to Cutbank. Cutbank would be quite a ways out of your way. A direct line might cross the Canadian border somewhere in Minnesota or at Pembina, ND, or??
Then again, weather will drive most of those decisions for you. No weather to worry about? (Like that'll happen!!) Punch in Alabama direct to, and follow the pink line...
Great little towns out in the Canadian prairies, nice folks and very accomodating. Gas isn't cheap, but accomodations are.
Coming south the last time, weather forced me south from Ft St John, and got me stuck at Red Deer. I slept on a flight school couch in their offices, and walked the three or four blocks to a restaurant to meals. Was going to set up a tent, and they said "no way--camp in here..."
So, pick yer pleasure. Lots of great places to visit in this here continent.
MTV