My sister has had her sierra for a long time over 250000 miles I say keep the truck. As long as you take care of the truck it will get you were you need to go for a long time hahah.
Keep the truck! its more useful and probably wont get much different fuel mileage. Ever tried to haul a fridge or something like that in a car, it doesn't work well. Plus, I don't care who you are, trucks are cooler and can pull more weight and trailers.