|Drywall is the material and 'sheetrock' is only one brand. It doesn't matter what 'brand' you buy. I haven't ever found any difference between brands except a few pennies. In fact when you order a large quantity of it you will get a mixture of brands even in the same size.
There is a difference in the thickness of the sheets though. there is 1/4" and 3/8" sheets that are designed to go OVER other plaster or drywall. Then there is 1/2" drywall, which is used as new walls most of the time and is the most popular. 5/8" drywall is a firecode brand and is used in kitchens and garages that are attached to the house.
Home Depot or Lowes is a good place to buy a small quantity but I prefer a building supply that has a boom truck to deliver the sheets right to the floor through the window. In fact the delivery people will remove the window sash and stack the sheets as part of the delivery! Now that's service!!