I really believe it is synergistic, but diet has more importance between the two. Personally, diet alone gets me pretty lean, but if I manipulate my energy systems work correctly, I can get even leaner. However, I have also experienced that even if my training and cardio is spot on, but my diet is shit, I don't make the progress I'm capable of. I see girls who spend ample amounts of time doing cardio and even train with weights a little, but then they go home and eat chocolate. What's sad is that they are perplexed as to why they aren't losing weight or "toning up." I think Chris Shugart said once, "It's your diet, stupid."
Sorry for giving you both sides of the debate, but I suppose the take-home message is that, in my personal experience, weight training and energy systems work only take an individual so far. Whether it's leaning out, bulking, or maintaining, a healthy and CONSISTENT dietary strategy is the primary indicator of whether someone is making progress or not.