The double shot is best for the challenging stages, but it makes your ship twice the size and easier for the aliens to hit.
I also suck at Galaga, but I still love it. I think thats why its still one of my all-time favorite games.
And after playing Atari's 1983 Galaxian, and how predictable the alien attack is. I'd love to see a Champ games version of that classic too.