Guardian: Specialized in group fights where defensive capabilities are more important.
Avenger: Specialized in solo fights and against heavier ships like bombers, freighters, transports and Gunboats. Its strength lies in its firepower and armour, making it ideal for using heavy weapons (mini-razor)
Avenger is just a I'll-go-down-in-flames-but-I'll-really-hurt-you-and-your-stinking-friends-in-the-proces-fighter in groups fights. In duels, I dunno. Never duelled in mine for some weird reason. I imagine it could do well with some early turning and side strafing to get that vertical fin out of harm's way.