Whatever inspires you most first! If you get to do both at once do that.
If you are choosing to focus on one first, start with the diet. I know people who run the iron man who know next to nothing about the quality and bioavailability of the foods they are regularly putting into their bodies. They focus all their time and energy on building muscles without considering what will be feeding those muscles. The idea for many is, if I just excercise enough I can eat whatever and have no problems. Well tell that to the many very overweight people who run marathons and would love to lose that weight. They are running marathons!
How about the many many people on successful diets who never excercise and lose a hundred pounds. They lose one hundred pounds all by focusing on their will power to choose what they will put in that biggest hole in our face. If you really want to lose weight the bigger pay off is in focusing on what you will and what you won’t put in that hole. You have will power and it grows with every little win.