Myself, I'd take a Challenger or a Cuda.
Oops, those weren't in the offering, were they?
Sorry bout that, I just couldn't resist.
To me, it would depend if you are talking "Old" Mustangs & Camaros" or the New Generation ones. Old Style Camaro wise, I'd be looking at the '69 or '70 and being a Big Block Guy, I'd have to go 427 or454. Old Style Mustang, Boss 302, Boss 429 or Shelby GT 500
New Generation, to ME, the Camaro doesn't have the Retro look down as well as the Mustang (or Challenger, for that part) The New Mustangs seem to be available with some pretty hot power trains and I have heard alot of comments that the Mustangs are built alot better.
Just MY 2cents, along with what I have heard from sources.
(No offence intended, either way.)