There is no guarantee that a bank will get you a better deal than a broker. There are times when it may work out better to use the bank, but there are also a number of times where going to a broker would be a better choice.
The best thing with a broker is that they generally have access to a number of different lenders and programs. Why is this important? Some lenders will lower there rate on 1 or 2 particular loan programs to corner that particular part of the market. A bank may not be that lender and hence you may have a rate .25% - .5% higher. Also, if you find a good broker, they will help you find the correct program for your current situation that is also in allignment with you future goals.