I've owned Turbo (2.1L and 2.3L) Volvo's from 1981-2009 and I know how to get the best mileage out of one. Rules one and two, stay out of boost, keep the RPM's under 2,500, and you'll get the best fuel mileage.
What people don't understand on modern turbo's is when the boost goes up so does the extra fuel being dumped into the cylinders. Not just to compensate for more air being pumped into the engine but that extra fuel is also used to cool the cylinders.
You might be driving a 2.3 liter Eco Boost but spool the turbo up and you might as well be driving a NA 3.5 liter as far as the amount of gas being pumped into the cylinders.
You were asking between the two which car would be better? I would go with the Mustang for the simple reason that if you planned on using the car on the highway it is far easier to swap out the rear gears for better mileage. I think it comes with 3.31 rear gears and you could swap those out easily for a set of 3.08's.
The problem I have with the 1.5L Honda is the motor is undersized for that car. Toyota learned that lesson with the Prius 1.5 liter and went to the 1.8 liter and upped the fuel mileage and power in the third generation.
My opinion for what it is worth is that 1.8-2.0 liters is the sweet spot for power and economy NA or Turbo. Go below the 1.8L range with Direct injection and Turbo charging and you're only looking for trouble in the "long run" with engine reliability.