In terms of raw power, Black Bolt can get his most powerful attack faster [i.e. scream] than Thor can gets his [i.e. godblast], and it's also repeatable and stuff.
In terms of relative showings? Their showings against Gladiator? Hulk? Namor? Thing? Thanos? This....would be an interesting discussion, tbh.
thor handled bb fairly easily when they met and he was hardly going all out. he was able to sense bb's weakness because of his connection to energy. even without the weakness detection, i'd take thor for certain here. that doesn't mean thor can't be put down by a scream though. be interesting to see if mjolnir could absorb the scream's energy. bb is very versatile and has some really solid energy manipulation feats himself, but thor is a rung above him on the tiering ladder. the scream has some great showings, and some that aren't quite as good. is it better than the godblast? maybe. pretty close overall i think. obviously bb has nothing on motherstorm, but thor's lightning can be uber as well. the hammer is too much as a plot device for me to say bb>thor in overall power.