So let's say Gigear can take two hits to Gigigear's one (which is fairly likely. You'll want to switch in to something Gigear stops, say, special Venasaur, or a toxic staller. Then they'll switch in something that can take care of both of them, but Gigear will most likely be able to take it while Gigigear won't.) That's 654 speed and 568 attack (still with great defenses) versus 612 speed and 639 attack with mediocre defenses and a far more crippled HP. Of course, both seem quite effective, but when you think about their limited movepool, staying alive seems far more important than hitting hard..
(Also, what EVs and Natures were you running with those calculations, Thorhammer?)