Downloading at the moment, but am still confused on the qualitative/quantitative differences between Black Mesa and Half-Life Source...? Like, for instance, what makes this better than HL Source?
Half-Life Source is just the same HL game but ported to a newer engine. (same textures, maps, environments, etc..) Except for the underlying engine there is no difference between HL and HL:S.
Black Mesa however is a complete remake (re-imagining?) of Half Life. The modders basically re-made HL from scratch. They made new models, new maps, new textures and added certain creative liberties (like there is a new window and office in the short hallway where Barney lets you in right after you get off the opening tram ride) so while it's not exactly the same, it's close enough.