Hi Andy,
In Steve's absence, I'll take the liberty of replying...
The Charger was launched in 1964 and was based on the Coronet platform. It reached the peak of it's popularity and prowess with the 1968/69 model years (Dukes of Hazzard, coke-bottle model) and offered with 318, 340, 340 six-pak, 383, 383 Magnum, 440 Magnum, 440 six-pak and 426 hemi. By 72, it had got much bigger, heavier and less sporting - and lost the beautiful flying buttress rear roof line.
The Challenger was a new model launched in 1970 (a bit late) to answer the Muatang/Camaro ponycar battle. It's Evil Twin, the Plymouth Barracuda was the more popular and sporting varient, with shaker hood, bumble-bee stripes and garish colours. Came with similar engine options - the hemi-cuda is legendary and faster than a Ferrari Enzo over a standing quarter.
The Challenger was a more luxurious 'businessmans' sports coupe - had luxuries like carpet, 8-track and electric windows.
Challenger/'Cuda very nice cars and the most desireable of the Mopar muscle cars of the early 70's.
A 426ci hemi-Cuda sold for $375,000 at this years Barrett Jackson auction in AZ.