I know this is an old thread, but I just wanted to add my comments.
I have found the F15 without self leveling suspension to be useless. I tow project cars every now and then. I find that the rear squats down a lot as you can see in this picture.
I used to have an E70 35d with rear self leveling, and I never had this problem. My 35i has no problem with the weight I tow. It's rated for 7500 pounds with the invisihitch. But I certainly miss the torque of the diesel when towing.
Selling my F15 now to get a diesel with self leveling.
If you are going to tow a jet ski, or a 5X8 utility trailer with garden waste, it should be fine. But for towing big boats, cars and big trailers, get the self leveling.
