Other than under your heel, I've not found much to choose from regarding the backings. I think I'd go with the latex - rubber will fragment under your heel and - depending on how much you use the car, hessian will also wear into a hole. The latex will also prevent any spills going through.
Not suitable for your car, but the best carpet I ever fitted into a car was heavy duty office floorcovering carpet. Extremely thick and mouldable - did a great job on my old Morris and provided excellent soundproofing as well even tho it was far from original.