First off the 2010 GT had a 4.6 NOT the new 5.0. Check KBB for value but remember that the custom paint and hood actually LOWER the value of the car. It sounds backwards but it's true. As far as what you should get it depends on what kind o car you need or want. Do you like the mustang but want something with better MPG? Look at a 2011 or 2012 Mustang
V6. 300HP and 31 mpg.
Need a truck? the new F-150 or new hemi dodge rams are all fine options, and the rams have some VERY nice interiors (specially for it being a truck)