The avenger is a better ship for a number of reasons, for one it forces a pilot to rely more on a fexible gun array rather than expensive missiles.
Back in the Vietnam conflict, missiles cost millions of dollars a kill while gun kills cost hundreds to thousands of dollars.
The 2 leech guns, 2 HV Mass, 2 HV photon, 2 stormfire, a mass driver turret, a tractor beam, 4 torpedo and 8 missile slots, better speed and turn ratios make the ship much more usefull than the Longbow, as it can kill capships and fighters with equal ease.
The Longbow has a better gun in its turret, slightly thicker armor, and better sheilds. These stats are offset by its increased sluggish handling and speed. So that leaves the added missile load, which is formidable, but the ship will likely be shot down before the missile load is shot off.
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Don't look conspicuous-it draws fire.