I'm just going to say this up front. I personally don't know. With that, I can say that I have the standard package ltz and never had any problems pulling anything. Anything being a 80 KW cat generator (which weighs more than a couple of cars), and skid steers on a regular basis, near 10-15k / year. Terrain may be a factor tho.
If I had the option I would go with heavy duty, just for the hell of it. Who know, you may need it down the road....