I am a firm believer that you can’t do one without the other.  Trust me, I have tried skipping one and doing the other, and results never show up the way I want.  It’s sort of not fair.  But, post baby, if you’re too tired to do both, I would suggest breastfeeding (burns more calories, and something about it being healthiest for babies too), and getting your bootie to the gym.  Start to firm things up, and then add in diet as soon as you get more sleep.  Hahahaha….