Assuming that you don't have a very good fix on what offensive types the alien ships are going to have
Well, it is best to tailor your force to your foe, rather than having a generic design. But marching on with this as a premise...
which would you rather build? Can you explain in terms of what happens in combat why you would select one over the other?
Whether I had super warrior ability (only Arceans and Custom Races) and my logistics size would probably be the biggest determining factor. I'll just assume you're not Arceans for the rest of this... but if you are, go all offense all the time.
As soon as I started to run into enemies though, I'd need to quickly find out their fleet composition to know what to build. Here is a basic decision tree:
1. Can the enemy fleets I face destroy me in
one round? Note, that's round, not overall battle outcome... but do they destroy you with just one set of rolls. In your example, do their fleets pack roughly (20+(8x2))x"number of fighters in avg. fleet" in offense.
If yes: Build the all offense fighter. This way you will do at least some damage back to slow down the enemy as they toast you one after the other. If no, continue to question 2...
2. Can the enemy fleets I face soak up all the damage my defense fleets can kick out? This is rare by the way. In DA though every now and then you will run into the Krynn or Iconians who are using larger hull sizes and went with defense. Figure it thusly: Is the defense on ONE enemy ship > 9/2x"number of fighters in avg. fleet". Say your logistics allowed for 3 fighters. They will do on average 4.5x3 damage in one round of combat, so if enemy ships pack 15 or more defense PER SHIP given these sample numbers your answer to this question is yes.
If yes: Build the all offense fighter. You will need this just to do any damage. If no, continue to question 3...
3. Can I build a ship that can destroy an enemy FLEET in
one round? Again, not battle, one round of shooting weapons within a combat. Given your small sample numbers, this will really only come into play if your enemies are using tiny fighters and cargo hulls with limited logistics ability. Later in the game though this question becomes very important. Trading individual ships for whole fleets is the key to winning the production war against the AI at high levels.
If yes: Build the offense fighter. If no, continue to question 4...
4. (a) Am I on offense and (

can I build a ship that can destroy an enemy ship with orbit bonuses in one round that (c) also has a higher attack value than the orbiting enemy?
If yes to a, b and c: Build the all offense fighter. In this case you have created an invincible attack ship that you can use against any number of enemy ships while on offense. The AI rarely uses orbital fleet managers, and if you have the higher attack value and can kill the enemy in one round you will always survive with at least 1 HP. Its hard to beat invincibility. This is very common late game. If no, continue to question 5...
5. Can I build a single ship with any offense that also has defense greater than enemy FLEET attack totals divided by two? That is, for example, if enemy fleets pack only 20 attack and you can build ships with at least 10 defense and still have some sort of gun, you would answer yest to this question.
If yes: Build the defense heavy ship. It will be close to invincible, and will at the very least greatly reduce your attrition. Some people prefer anywhere from 60% to 75% of enemy fleet attack totals for their invincible ship calculation... but even at 50% you are going to significantly reduce the damage you take. If no, continue to question 6...
6. Can I get at least 1/1/1 for my defense stats at a reasonable cost efficiency? I bring this up because the first kind of armor plating you get is way too large and expensive for the one point bonus it confers. Otherwise these are the three best points you can spend in defense in the whole game. Because of the square-root applied to off-type defense, only the first point in each catagory will hold its value against every weapon in the game.
If yes, design fighters that include at least a 1/1/1.
From that point on its a matter of taste and what technology you have researched. How fast do you want your ships to go? How much beyond 1/1/1 in your best defense tech do you want to increase that one defense type?
Generally speaking - if the average hit points of the ships you build are roughly greater than or equal to the firepower of enemy ships you face, you will do better with defense, because you have the hits to survive multiple rounds and make that defense pay off by recharging. If, conversely, the average hit points of the ships you build are less (or vastly less) than the firepower of enemies, you want to stick with attack. Because late game weapons eventually far outpace HPs, it generally tends to be the case that late game an all offense approach works best (plus the tie rule). Because early game weapons aren't so hot against even medium fighter HPs, early and mid game it is often better to build more balanced craft.
Hope that helps,
~ Wyndstar