Keep in mind that Armor Technology is used vs Interplanetary Missiles... Both are needed if you want to build a solid turtle, but as for which one is more important, it's hard to say. If you don't have a high enough Weapon Technology, your defenses won't deter players from attacking because they won't be getting a ton of losses assuming you have Plasma Turrets. But Armor Technology reduces losses vs IPM parties which is pretty common to soften turtle shells.
Being a turtle is a defensive strategy (duh, obviously). When a turtle reaches a certain size, it has nothing to fear from speedfleets. Once that size is reached the turtle can only be brought down by RIPs or IPMs.
The primary goal of any turtle, is to be unprofitable. To make the effort and cost of bringing down that turtle far greater than the potential benefits of such an act. Once the turtle is no longer threatened by speedfleets, weapons tech loses some of it's import. That is not to say that you should neglect it entirely, nor do I try to downplay how very important it is against speedfleet. But you only have 2 ways to combat IPMs. More defences equal a greater cost to remove it entirely - which sometimes works, sometimes don't. The other is armour tech, making it more costly to remove an equal number of defences - and that is a real pain in the arse for anyone contemplating IPM'ing your defences.
I might just add that shield tech is fairly important as well. It greatly improves the survivability of RIPs, which as we all know is a key component of any major defence.
Long story short, you want higher than average techs as a turtle with a slight focus on armour tech.
I can certainly say that my 2x turtle benefits greatly from my 20/20/22 techs.
All combat technologies are important, Weapons technology makes the most out of your Gauss Cannons and Plasma Turrets, Shielding technology makes the most of your Shield Domes and Ion Cannons, and Armor technology is important for minimizing damage from IPM's as mentioned above.
I'd suggest have combat technologies at level 17 minimum, and get your Missile Silo up to a minimum of level 10, that gives you 100 ABM's, and would mean someone wasting 100 IPMs on nothing if they were to attempt a mass strike.
As for defense, the numbers are limitless, the more the merrier, with a good balance recommended.
Helllloooooooooooooooo everybody, wow it has been a while since I asked this question. It's interesting to see now what would be the best option.
So how far did I get by now:
Currently I have:
And my techs are now:
So in essence there is already quite enough regular defense, and if I had to choose to upgrade on of the Technologies, I would think about Armour, since any regular attack with speedfleet is unlikely. IPM's can be used, I have rocket silo 12, but this does not really help if somebody REALLY starts to throw with IPM's (by the way, which are still way too much overpowered imo). So this is a difficult one, I know that if somebody wants to get this down with IPM's it will costs them a liver and 4 kidneys, but there is always the possibility that someone will come with an armada of Death Stars. So that makes Shielding Technology the better choice. I also have to look to the costs of these technologies in stead of building more 'own' deathstars. For example, would the effect of Shield Tech level 19 be more than the amount of extra Death Stars I could build of the costs of that particular tech? But at this point the cost of Shield Technology level 19 is pretty doable, only 52M Metal and 157M Crystal. I think I might consider that. 
