It's the same stuff. Except, BRS also offers magnesium chloride, which is probably better for long term use. This is because sulfate ions can build up and quickly shoot to levels many times higher than natural sea water. And BRS also offers sodium carbonate, which is better if your tank's ph tends to run low. Baking soda, on the other hand, is sodium bicarbonate, which is best if your tanks ph tends to naturally run high.
I've been using the DIY recipe for a year and a half with great results. And it's cheap! That's right. My alk supplement brand is Arm & Hammer. Haha