mustang, the "charger" is just a 300 w/ some different body panels....though unlike the mustang since it's not a 2 door coupe (like it's namesake was) you can actually fit people in the back of it,lol
so really it depends if you want something powerful but big and heavy, or slightly smaller but more sporty....and also how many people you're gonna have in it most of the time, cause it's well known that the mustang's rear seats aren't good for any humans past maybe 12yrs old,lol
What are you going to do with it ?? If you're going to drive it, then either, but it you're going to immediately pop the hood & start tinkering, then the Mustang. There's a HUGE aftermarket following on the Mustang, everything from billet trim pieces to turn-key engine upgrades.
The Charger is a 4 door, but from what everyone has told me that owns one is they're truly a pile of junk, people think they're cool because the have a "HEMI" but in reality that engine is garbage. Buy a Mustang GT 302 Boss.