I'd say it all depends. I generally wear chaps of some sort anytime I ride, I like the extra grip. And hey, chaps just look way cool!
That being said:
- Shotguns hold in the heat - maybe a good thing up north?
- Chinks help fend off tall brush and light rain but not much else (although they look way cool)
Another thing to think about is the material for your chaps. The purists here will cringe but sometimes leather may not be the best bet as even the best waterproofing will eventually soak through.
Here in the PNW I tend to favor a set of heavy oilskin shotgun chaps that turn aside our incessant rain. Filson makes some excellent, but spendy, oilskin chaps that will keep you dry and warm.
What type of terrain, and associated climate, will your trip from Canada to Mexico take you through?