So two units of 5 Havocs with 4 Missile Launchers and Flakk Missiles comes in at 175 points a unit, or 350 points for the two of them. So you get 8 shots at a Flyer a turn after it has had a chance to shoot you, which nets you 5.333... hits and 2.666... glances and penetrates against AV11, so slightly less than a single Flyer in one turn.
Two units of Havocs with 4 Autocannons are 115 points each, or 230 points together, and then the Aegis Defence Line with a Quad-gun is 100 points on top of that, coming in at 20 points less than the Missile Launcher Havocs. The major benefit of this kit is that you get to shoot at the Flyer when it comes in and before it even has a chance to shoot at you. The Quad-gun being fired at BS4 with TL will get you 3.444... hits alone and 1.7222... glances/pens. Then on your turn the 8 Autocannons will hit 1.333... times, getting you another 0.666... Glances/Pens against the same AV11 vehicle. This all comes out to slightly less glances/penetrates than the Flakk Missiles, but it does come in at 20 points cheaper and gives you vastly superior firepower against transports and other lightly armoured vehicles.
With that all in mind, I think I'll be going for the Autocannon Havocs and Aegis Defence Line every time because while they do slightly less against Flyers, they also cost less, do far more against transports and infantry and the Quad-gun gives you a slight chance to take the enemy Flyer down before it even gets a chance to shoot.
Just some food for thought.