In general, if it's something like a weapon, or some other thing that does stuff to other objects, that makes more sense to have as a ship part.
I couldn't agree more.
On the other hand, stuff that affects the way a ship responds to actions taken against it, such as recovering damage, or absorbing SR weapons, is something that feels like a better fit for hull effects. It makes sense that a hull has some special properties that allow it to reflect weapons, or fly into a sun, but less sense that it has some special property that allows it to turn a planet into an asteroid belt.
Hm... I'm a bit wary when it comes to incorporating too much effects/abilities into the hulls, instead of making ship parts that provide these effects/abilities. You loose flexibility when designing ships (because the "mix" of abilities is kind of "hardwired") and it increases the amount of hull types.
The ability to absorb SR weapons for example could be implemented as a special armor ship part instead of attaching it to a certain hull type (and thereby restricting it to this hull type).