AD, my missile use case scenario for nukes is pre-ND. That also reduces the armor possible to be mounted to counter effects, and the spinals are less efficient fed by larger power plants.
OK, at lower TLs missiles is the most important weapon, but spinals are still the only way to permanently disable ships. As missiles dominates combat it becomes even more important to have heavy armour; expect more planetoids at lower TLs. You can't really have good armour and good agility, so you have to choose one of them.
The Eurisko fleet is an example of what happens at TL-12. It presents the enemy with a battle-line of armour 18 rocks with missile armament only. The fleet has a few spinals too (IIRC seven spinals vs 70+ missile rocks), but they are less well defended and would be scraped immediately if put in the battle-line.
The first part of the battle is slowly grinding down the enemy missile line by hopefully inflicting more weapon hits than the enemy can repair. Then when the battle-line is about to break, you place your spinals in the battle-line to exploit the breakthrough and kill damaged ships in the reserve.
The other alternative is agility 6 armour 4 missile frigates that are difficult to hit, but easy to damage. More vulnerable to PAs, less vulnerable to mesons.
You need many small missile batteries to generate lots of hits against slow rocks, and many large missile batteries to hit small, fast missile frigates. No single choice is best against all opponents...
