Practice both methods.
There are distinct advantages to each method. Presentation is usually a tad faster with the thumb sweep method, I rip fewer shirts using my off hand. But there is a definate place for each. Like any defensive technique, don't be a one trick pony.
With the thumb sweep method, catch your thumb on the hem of your shirt and pull it up, WAY up over the pistol butt. Then reach down to your handgun. Draw the pistol normally then, pulling it up as far as possible then rotating it toward the target.
With the off hand grab, pull the shirt up as far as you can, then draw the pistol up as far as possible, then rotate it toward the target.
I mention pulling the handgun up as far as possible because the stories about guys flinging their handguns downrange are usually because they haven't actually cleared the holster and the barrel catches and pulls the butt out of their hands. Don't be that guy. It's embarrassing as all get out at the range and downright life threatening on the street. Pull it up, then rotate the barrel toward the target. This puts you in a position to begin shooting immediately if you have to.
I also agree heartily with the poster who said to not get in the habit of firing every time you draw. It should be a concious decision. When establishing your firing grip with the gun still in the holster, place your finger along the slide. Keep it there until you decide to fire.