I've got both the Thunder and the Dark Thunder. The original Thunder was/is a flare monster, no doubt.
The Dark Thunder flares considerably less, at least for me. It's a strong coverstock, to be sure, but it's predictable, and it finishes very well. The original Lightning was for up-the-track players, but I think the Dark Thunder could be drilled to play that line.
My Thunder flares 5-6"; my Dark Thunder about 2.5".
Jess