Cuda:
3-400 lbs. lighter than Challenger. Like having an overweight person in your car all the time.
Need I say more. 
http://wcuk.files.wordpress.com/2007/07/225629_fat_guy_in_car.jpg
Offset by the sloppy aerodynamics of the rather stubby Barracuda. The extra length of the Challenger, working with its far smoother lines, more than makes up for the slight weight difference. That silly angle along the rear side window causes measurable turbulence at speed. The gap between the header panel and the front of the hood also contributes to the aerodynamic sloppiness, as well as reducing ram airflow into the scoops (assuming the hood is so equipped), negating part of the cool-air benefit and costing power.
Its entirely possible the lighter 'Cuda will get off the line a smidgeon quicker, but the Challenger will quickly overtake it as the Plymouth plows into its own wind shear and pressure waves.
That said I'd take a 'Cuda over anything _but_ a Challenger. There is no shame in being second best to a car so perfect as the Dodge Challenger.
