I found this statement by CAPT Dick Hewitt from the 82nd Squadron, 78th Fighter Group, Duxford, England:
"Webster describes a Mustang as 'a small, hardy, horse of the western plains. I'd say it was more like a 'pinto pony with the power of a Clydesdale. I was often asked which of the two, 'Jug' or Mustang I thought the best was. Each did the job they were designed to do. There were times I wished I had been in the other's seat but I'd never, ever downplay either. Unless you have been there and done both, best to keep a 'zipped lip.' I'll just say: Thank you Republic Aircraft and North American Aviarion. Both were the main reasons I lived to tell the tale.
The debate usually favors the P-51, but I like Hewitt's take.