If your going used, look and drive a Ram MegaCab. The interior room will blow you away. GM has one of the most dated interiors until you get the the last couple of years. Ram has one of the nicest. Fords are simply overpriced. A gas motor is fine for your smaller TT. So unless your going much bigger in the next couple years get a gaser. Diesels can be a bit more expensive for oil changes, but get better fuel economy. So its a wash in my book. Besides diesels tow way more and better than a gas motor.
BTW your dad is way behind the times. Big block gassers were the only game in town 20 years ago. A lot has changed since then. Modern diesels are great motors with lots of pulling power and great fuel economy.
