Some comparison data:Wife and I have settled on either a 2012+ F150 with the 5.0 or a 2011+ F250 with a 6.2.
~15.5 mpg for the F150 and 5.0 :http://www.fuelly.com/car/ford/f-150?engineconfig_id=302&bodytype_id=&submodel_id=
~11.5 mpg for the F250 and 6.2 :http://www.fuelly.com/car/ford/f-250_super_duty?engineconfig_id=299&bodytype_id=&submodel_id=
That's more than I expected... but could be because 250s are usually "working" and 150s aren't.
The F250 (and other 3/4 ton+ trucks) are exempt from mpg standards and are made for work, so mpgs are down the list of priorities. 1/2 ton trucks have become primarily large, do-everything commuter cars.
I'd get the F150 if you won't be towing or heavy loaded, and the F250 if you are.