Hey, calm down!! I got used ones too...
Seriously though... I dunno where you're getting your mileage figures, but It's been my experience (10 years in the used auto business, specializing in trucks) that the V-6 will get within 10% of the mileage of a midsize truck for the same trip and BETTER mileage when hauling that load.. (a GM I4 manual has a total trailering capacity of just 2400lbs as opposed to the full sized V-6's 5400 lb capacity... so when you ARE hauling OR traveling over 55mph you'll be cranking twice the RPM's)
Since you are buying used I assume longevity of powertrain life would come into play... The full sized truck won't work as hard as the midsize, that should lend to a longer (more maintenance free) life... If you're less than stoked about a truck getting truck mileage, imagine how incensed you'll be when that little 4 banger blows up from being overworked..
Just trying to give some solid advice....
(and now that I've advised you I simply must insist that you buy your vehicle from me at a maximum profit margin :rubs_palms_together:)