Kind of like Rocky Balboa vs. Apollo Creed?
Adding to what Rúmil just said- 'Mighty' is kind of a nebulous concept. If you asked "Who is mightier at forging rings of power?", then I'd probably answer Sauron. If the question was "Who is mightier at reading large numbers of books while maintaining a high comprehension level afterwards?", then I'd say Gandalf. If you asked who would win in hand-to-hand combat, I'd say who knows, neither one of the two are combat specialists. However, at the end of the third age Gandalf had been out beating on Balrogs and Nazgûl, while Sauron had been sitting on his throne, so I'd say Gandalf was in better fighting shape (unless there was a gymn in Barad-dûr)!
Again, re. Elu's quotes, when Gandalf says 'Black is mightier still', this does not necessarily refer to Sauron alone, but perhaps to all the forces that were arrayed against the free peoples. It is also somewhat modest, as the victory of the free peoples over Sauron and his minions was due in no small part to the actions of Gandalf- Black was certainly more confident in it's strength, but it was overthrown by the 'white' all the same.
When Gandalf says that the Dark Lord is more dangerous than himself, this again is a strange concept to get a hold of. Gandalf might be very powerful in combat, but he is not likely to employ the sort of tactics which Sauron used in Numenor, for example. He could therefore admit to being 'less dangerous' without saying who would win in physical combat, which is what I believe this thread is about.