If possible, go to the range and rent a Glock 19, a Smith M&P, and a Springfield XD. Try to find similar sizes and barrel lengths to avoid apples-to-oranges comparisons. These are all ~$450-$550 guns, and have reputations for simplicity, accuracy, and reliability. Night sights will usually add $75-$100. Also, lots of aftermarket sights, holsters, and other accessories will be common for all three, especially the Glock.
A cheap gun doesn't always mean a good first gun. $600 will allow you to find a great first weapon, and you may never feel the need for a different one. I have found that a more expensive weapon is not always an upgrade. BTW, if you decide on the M&P, I would strongly recommend finding one that DOES NOT have the magazine disconnect safety option. These are rarely in stock, but ordering one should not be a problem. I may get flamed for this, but the magazine disconnect is a terrible feature, in my opinion.