I don't know about the Prestone stuff, but I have used the GM/Bar's leak tablets with very good results.
My Oldsmobile radiator just sprang a leak the other day, and I decided to use the Subaru coolant conditioner, which is actually Holts Radweld. Cost me $2.07 out the door at my local Subaru parts department. Put one 4.4 ounce bottle in, and drove it to a Shell gas station about five miles away with the heat on, and the leak had already stopped when I pulled in to fill up with gas.
The Holts Radweld is made in England for Subaru, and that information is on the back side of the bottle. I did not want to fool around with crushing the tablets, and waiting for them to dissolve in warm water, which is the way I do it. The Subaru/Holts is already in a concentrated form, and all I had to do was shake it real good. It was also cheaper than the Bar's/GM tablets too, which surprised me.