I use nearly all bombers, thourgh normally i have a defense fleet at primeary defense points so and have verry few things live long against 53 bomber wings. those don't die when the fleet shows up

4 hangars or 24 turrets backed up with repair plats? Admittedly, its cheaper to build hangars and strikecraft are replaced free of charge, but in Sins defense is already very very weak. At least with a few worlds with Phase inhibitors and masses of turrets n repair plats can hopefully hold the enemy back long enough for your main fleets to come to the rescue.

It depends what stage of the game you are playing at.

Early game when its only pirates your worried about, bombers will take out the pirate ships quickest.

But if your being attacked by an enemy fleet that has massed fighters, then your bombers are vulnerable and you need fighters. So mid game you need about a 50/50 mix. Late game your probably better with more fighters than bombers, as the enemy fleets get larger, but it depends on what they are composed of.

The other thing is that Bombers will target ships by themselves, where fighters need to be selected as a group and then told to attack ships. So if your busy directing capital ships, bombers are much better since they dont need direction to attack ships. Left alone, once all the enemy fighters and carriers are gone, your fighters will swan around doing nothing while there is still a significant fleet to attack, while the bombers are in there killing ships.

So, against pirates, a mix of 2/3 fighters and 1/3 bombers would be ok?
In all my games so far, the AI opponents usually use only few planet-bombers. Only the pirates tend to use a larger number of bombers. It is annoying when they beat the snot out of my planet because my cavalry was busy somewhere and could not come in 'time'. Then, if my fighters form Hangars manage to kill the pirat bombers quickly enough, my planet is safe. It takes quite a lot of pounding to destroy installations when there are a couple repair platforms healing damage in the area.
I am always keeping a keen eye on my enemies. What gives me fits are the annoying pirate attacks because they (the pirates) have the ability to 'conjure' a strike fleet out of thin air. And, in most cases, one of my planets is perched right on top of the pirate base. Only one, and a short phase lane is too little time to bring my fleet there in time.
On the other hand, the AI cannot just 'spawn' a dangerous fleet just under my nose. Usually, i will see it coming long before they are able to place a strike on me, and send my best ships to shatter their would-be conquering fleet. I will not wait for them to come at me like i am usually doing with the pirates.
Well, up to test the 2:1 setup for a while, it might be what i am lookig for. Thanks for your replys, keep them coming!

Against pirate raids, bombers are most effective against their heavy cruisers (Pirate Corsair), fighters are effective against their siege ships and their light frig (Pirate Cutthroat). Neither are very effective against their flak ship, though they'll eventually kill them too.

The AI does like going carrier heavy (I'm still using 1.05), and more often than not, there are more bomber squads than fighter (like a 60-40 split).

apricot: Last I checked, you can't create a cease fire or peace treaty with the pirates... though I sometimes wish I could...

I don't know what you are talking about, you can always see pirates' raid route.  You can attack the group as they flying by your planets.

I play a blitz style game. The structures still work even after the planet is destroyed so I normally lump a massive amount of defences in front of the planet around all my buildings. No hangar defence because you can have a lot more turrets instead. If they manage to bomb the planet the only thing I lose is a small tax income, most of my credits come from tradeports anyway. If you have a colony ship you can just send it to get the planet back. I can normally take 5+ planets in the space of time it takes for the enemy fleet to destroy my 1 planets defences. I use carriers in my attacking force and I usually use bombers solely so I can snipe enemy capital ships as soon as my fleet jumps in. Quite disheartening.

The structures stay there. But I not sure whether they're operational.

If you lose a planet, any surviving structures remain and are still operative. However, all your constructor ships go boom, as well as any unfinished structures.

... The pirate ships appear from phase space, like a interstellar jump, at the local pirate base.

Then they slowly go thru phase lanes and shoot up your planets. They tend to shoot up anyone in the way too.