Here is a link for a gas to diesel calculator.
If MPG is the only factor it might be better to stay with the 150, if you are wanting something more than an mpg gain (power, capacity ect) then diesel may be the way to go.
How is mpg when not towing and how much towing do you do?
I have a diesel and no one can convince me otherwise but that's my opinion and I do not try to blindly push it on anyone, I tow mountains and I like POWER and longevity. Do some research and make an informed decision no matter which way you go.
Fuel Cost Comparison Calculator