I have used both the Coopers and Muntons. I like the Muntons a lot better because you can add just enough tablets to carbonate your beer properly, depending on the style and the size of the bottle. The Coopers drops work, but it is one great big hunk of sugar that overcarbonated most of my beer.
To be honest, I have only ever used these in an 'emergency'. I still greatly prefer to prime with 2/3 to 3/4 cup of boiled dextrose. Easier and cheaper, IMO.