The chimera is much more manouverable and has a higher impulse modifier. So it speed tanks way better than the avenger. --it also has higher tier tac abilities. So it outdlys and outdamages the avenger.
Its tradeoff is 7 weapons and less hull/shields and the lack of higher end engineering boff abilities.
The Avenger is a cruiser wich just happens to be able to carry cannons. That alone wouldnt make it worthwhile. Its the 5/3 weapons layout and a ltc/lt tac combination it can do. Plus a console that basicly is another forward facing weapon.
The chimera is more flexible especialy with its dts console. It can act like a mini cruiser while the avenger can act like a big escort.