I worked my week of exercise out but it took me 6 months of trying different peoples routines and finding my own for good results. I'm now on a 50 minute muscle building session maximum and finish off with a 20 minute fat burning high heartrate bike based interval exercise 3x a week, and once again I'm starting to shed fat and get my muscles back in shape.
You can do both but ideally I would just build muscle but my gym partner and I both need to lose the fat