For combat, it's rather simple at the sub-million end of the scale: Sidewinder < Eagle < Cobra < Viper < Diamondback Scout (not sure whether the Adder is above or below the Sidewinder, but in any case don't use it as a fighter). The latter has speed comparable to a Cobra, agility better than both Cobra and Viper, better shield potential than both (with 2x A grade shield boosters, a Scout has stronger shields than a Viper without shield boosters), and the best of both worlds for hardpoint placement: all close together for fixed weapons, nothing below the hull or otherwise obscured to hinder gimballed weapons.
For anyone who wants a pure combat ship, I'd advise to save up for a Diamondback Scout directly, and skip the Cobra or Viper.