that challenger is a photo chop,and the mustang is an artists rendering.
i would lean more towards the mustang version as it looks more aerodynamic
the nose on the challenger has the aerodynamics of a brick and would get its ass kicked on the speedways if that truly will be the design. think back to the charger and its changes just a few short years ago.

i had also heard that it will be all done with decals too so until i see actual photos
of the cars i dont believe anything nascar does