I, for one, am firmly in the Hardelot camp and I do not see tree encroachment as a problem at Les Pins. Maybe some would feel that way, but the course is just too good for it to have any effect on me. The best thing you can say about any course that is, in fact, in a forest is: "It does not feel like playing in a forest." And it absolutely doesn't at Hardelot Les Pins. I haven't played the second Hardelot course, but it's an unheralded modern layout, so why would anyone forego a flamboyant Simpson for that?
That being said, Le Touquet has (IMHO) two great courses in La Mer (Colt) and La Forêt (Hutchinson). Both slightly below Hardelot, but there's two classic tracks and Hardelot has only one. Plus, there's a better selection of accommodation at Le Touquet and Belle Dune is closer. My recommendation would be: for a week-end go to Hardelot, for anything longer go to Le Touquet.
Interestingly, the weather can be a lot different there compared to Kent.
Ulrich