I used to do mostly weight training, and would only do cardio once in a while, or like a 10 minutes on the treadmill at the end of a workout. But I found I wasn't really losing any fat at all. So recently I switched and started only doing cardio. And a big change was I now do treadmill, elliptical, and stairmaster, about 20 minutes of each. It's a brutal workout, but I feel like cardio is more effective if I do it for like an hour.
Focus on whatever is your weak spot. For me, my cardio fucking sucks bad and I needed a lot of practice.