Gasoline vs. Diesel for towing?

I'm looking to buy a new truck to pull my 16 foot long cargo trailer fully loaded it weighing about 5000 pounds including trailer, cold winters. Normally not more then 100km a day with a 50/50 of city to highway. I don't know what direction to go for a truck looking to spend around 10 000 to 20 000. Any import on suggestions on models of trucks would be helpful.