The causeway would definitely be faster than through Terres Basses. Then take the Galisbay route around the outside of Marigot.

The other way is over the hill through the outskirts of Philipsburg, and is probably about the same timewise as going through Marigot. The hills really aren't a problem, and the traffic going that way isn't any worse than through Marigot, at least in the morning, when I presume one would be heading to Orient.