If you can afford it, buy a diesel. Yes, diesel is more expensive, but even with that, the efficiency of the engine will pay for itself with a few years. Not to mention lower maintenance costs, longer life expectancy, and much better operating torque. Throw in the fact that a diesel can be reprogrammed to put out far more power, or dialed down to increase efficiency, and the benefits far outweigh the extra purchase price.
If you are considering doing any towing at all, then diesel is pretty much the only way to go.
BTW, it is my considered opinion that there should be a law requiring all pick-ups and SUVs over 4500lbs to use diesel motors...