Gas Mileage Difference Between These Two Engines?

I'm going to be upgrading from a Chevy Impala to a Chevy Silverado 2500 or GMC Sierra 2500, so I'll be experiencing quite the drop is Gas Mileage There are two Silverados I'm looking at, a 6.0L Gasoline Model Crew Cab with a Leveling Kit, K&N Intake, MagnaFlow Exhaust, Electric Fan, and it's... show more I'm going to be upgrading from a Chevy Impala to a Chevy Silverado 2500 or GMC Sierra 2500, so I'll be experiencing quite the drop is Gas Mileage

There are two Silverados I'm looking at, a 6.0L Gasoline Model Crew Cab with a Leveling Kit, K&N Intake, MagnaFlow Exhaust, Electric Fan, and it's Chipped.

The other one is a GMC Sierra 2500 8.1L Gasoline Model, Crew Cab with a 6" Lift and AirBags for the rear suspension. No other mechanical upgrades.

Between the two, as an estimate, what would be the difference is Gas Mileage (city is my primary concern), is there a big Horsepower Difference? Being that these are both Gas and not Diesel, is it defeating the purpose of getting a smaller engine because it's working harder?

I would mainly be using for everyday driving and occasionally would haul a Four-Wheeler or trailer. So nothing rough, basically it comes down to if there is a dramatic difference in Gas Mileage?

Thanks in advanced! Any information is helpful.
Update: By the way, the Silverado is an 05' and the GMC is an 03'.
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