I prefer the cast iron because the sear is better and the control is better. Sous vide is great too, but for elk and deer it takes so little time that I generally only do the cast iron. I save sous vide for longer durations.
But, my favorite tenderloin and loin prep is carpaccio. Thaw, trim, refreeze a few hours to almost frozen, then shave thin and place on a plate around the perimeter. Arugula salad in the middle with a homemade ponzu type vinaigrette and then salt/ pepper to taste. Makes those precious cuts last longer too!