Mortgage interest rates are based off of the prime rate (the amount the Federal Reserve charges to banks). For this reason, you're probably not going to find a huge difference in the rate you'll pay from one lender to another (though some lenders will try to hide this fact by publishing a lower interest rate and then charging discount points to make up for it). You may find a slight variation in the loan origination fee charged by each lender, but it probably won't be enough to sway you from one lender to another. So what considerations are important in choosing a lender?
Ultimately, it comes down to your convenience. If you have a solid credit rating, a good mortgage loan officer should be able to tell you, within minutes, what documents you need to turn over to him/her for processing and have you approved for a loan within a couple of hours of receiving your application.
So your primary questions for a potential lender are: