They are quite different sounds, the Black Dog will give you the heavier mids that Jimmy Page used in his tone, whereas the Emerald will give you tons of harmonics. Having said that, you can get the harmonics from the Black Dog and you can fatten up the Emeralds with your amp choice. Another consideration is your Les Paul. If it is a dark sounding guitar then I would go with the Emerald, if not, then the Black Dog is a good choice.
Classic Rock covers a lot of territory, so try to pin down what you want in sounds. I have a PRS Artist II with Mules, a 1953 Les Paul conversion with Black Dogs and a custom made 25.5" scale mahogany Les Paul with Emeralds, and each of them sounds great, but they are all different and suit different things. If you are into classic rock, then the Emerald would be Gary Moore, the Black Dog is Jimmy Page and the Mule is Paul Kossoff.