as far as cardio vs diet, it really depends what your goal is.
it's all somewhat connected. it also depends on how your diet looks currently. for example, if you eat nothing but fast food and soda (and if you have a caloric surplus), you benefit WAY MORE from just changing your diet than you would benefit from just cardio while keeping the same diet.
if your goal is to build muscle, your diet will also be more important than cardio because a lot of it will have to do with having enough protein to build muscle and also enough calories to not reduce lean weight.
one thing i suspect but not sure about is that better cardiovascular health may help on your lifts and recovery between sets. I think MMS said it in another thread but cardio can be used to increase a calorie deficit (and also to improve your cardio of course). but the reason you lose weight/fat is because you have a calorie deficit. and that's directly affected by diet. cardio is kind of an indirect approach. like i said, it's all connected.
is one better than the other? it's depends on what your goals are and what your diet and cardio are looking like right now. but nothing stops you from doing both. just becareful if you're dieting and doing cardio, make sure you still eat enough or else you'll lose muscle and may injure yourself.