Are you stuck, not sure if you want to slim down or add muscle, why not do it at the same time? Weight training is one of the best ways to loose fat and also the best way to add muscle.
To add muscle size (hypertrophy) the trainee must lift between 6-12 reps per set. Well isn't that a coincidence, the same rep ranges, 6-12, is also prescribed for people seeking fat loss. Doing long periods of steady state cardio on a treadmill or your choice of cardio equipment are not best for fat loss.
If you are looking to cut fat and it is only being made tougher because beach season a just around the corner, don't freak out and cut a drastic amount of calories to lose weight. On the other side of the coin, don't go to the closest fast food restaurant because you want to bulk and look like Arnold Schwarzenegger on the beach. Both of these physiques take a lot of time and dedication to obtain. Adding a couple hundred of calories a day will help you add some extra mass or cutting a couple hundred calories will over time help you loose fat.
If you find yourself in this situation it is recommended, as stated above, to train in the rep range of 6-12 reps per set and reach muscular failure each set. This will increase caloric expenditure, produce hypertrophy (size), deplete glycogen stores (great for fat loss) and burn more calories at rest (resting metabolic rate). Adding some type of conditioning at the end of your workout like intervals will help excel fat loss.
Use large compound lifts like the squat, bench, deadlift, chin ups and presses. These exercises will requires much more muscles fiber recruitment rather than use machines or isolation exercises. Having multiple muscles firing during a lift will expend more calories and build more muscle helping you achieve a lean strong physique.
In conclusion, one is not to trump the other. It is totally based on you and your desired look. It is possible with proper training and nutrition to add lean muscle while loosing fat. The human body will expend more calories to maintain a pound of muscle than it would on a pound of fat. Lift weights, eat your vegetables and turn your body into a fat burning machine!