I asked these same questions a few weeks ago, and got a lot of responses in favor of the one body part per week.
I am a cardio freak, and this allows me to start with cardio, raise my heart rate, then do app. 15 minutes of weights at the end, when I am already warmed up and hr elevated.
So far so good -- I feel great, I am seeing some good definition, and I have not gained any weight [and maybe lost a pound or two].