Muscles burn more calories than fat while resting. However, this doesn't result in the spot-reduction you're hoping for. What it does is increase overall metabolism. It will burn fat equally all over the body. That said, increasing muscles in certain areas can help shape your body through toning. The muscles will become more firm as you work on them. If you're wanting to improve your tummy area additional sit-ups won't burn fat just from your middle, but it will tone the area a bit more. Another benefit is that if you increase the strength of your stomach and back muscles it will help you 'suck' your tummy in, as well as improving posture as you work on your core muscles. So you will end up looking slimmer even without losing most of the fat in your stomach region.
Hope this helps!