I'm guessing your not going to be pulling the horse trailer all the time. You'll get a little better fuel mileage with the 4.8 vice the 5.3 and a lot better fuel mileage over the 6.0. The 4.8 may not have as much power as either of those engines but dont count it out, those engines are great for towing and driving unloaded. Most trucks log the majority of their mileage unloaded making local trips. Unless your hauling Clydesdales I think you'll be o.k. with the 4.8, the fuel and purchase price you save will be well worth it.