Both are very important.
See aerobics not only helps you shed weight from all over your body but it also helps to get your blood flowing, heart pumping, and lungs working to help them work more efficiantly. It also helps flush toxins out of your system by throwing the lymphatic system into hyper drive.
Weight training helps tone muscle making you look leaner and helps with developing a strong core. Also, the more muscle you have the more calories you burn doing your normal activities through the day. Actually to be precise muscle burns about 50% more calories then fat through out the day.