The codex choice goes far deeper down the tech tree and changes alot of things how you progress through the game.
Red gives you an instant boost through the rough early game by offering free armor and guns. Questionable the weakest unique craft-gun (custom spike rocket launcher) but the tank-menace.
Gold gives you treasures, a great uniform + lash and the 'Flame Cannon' as the 2nd best craft weapon (burns hellerium instead of chems, that's why place 2)
Green gives you medical stuff, the healing gel is an instant upgrade for bandages and a free medic-suit. Also 3 additional hands. The craft charger laser looks weak but it doesn't cost ANY ammo so you can gun down all civilian crafts by simply pew pew them with the charger laser.
Grey is meant to give you alot of things to research (7 free disks, a free liber occultus) and a buffed up seagull launcher. It's the best custom craftgun imo and usefull throught the game until you can spam meteor rockets. It's meant to get voodoo faster but that's a really long way to your personal voodoo-school and with the newest update you've to look for 10 hypno panels to even build a voodoo school at all. Grey gives you no direct battle-field boosts but instead more intel and the most versatile ship among the menace-class.
Lady luck for you having not 1 but 2 syns at your disposal. I had about 6 non-transmitter space missions and not a single 'lost spaceship' among those.
Yes, enemy zombies take damage from the strix' corpse-explosion.