Hull technologies tend to increase both max hull points and hull regen rate...
Armor techs tend to only increase armor....
Armor is usually considered better because it is instantaneous...for example, if I have ships fighting on the front line and some of them are damaged, they will immediately benefit from the armor (as a form of damage reduction) but will not immediately benefit from max hull increases (since it would take time to regenerate that hull)...even undamaged ships still have to regenerate hull up to the new HP max when a hull tech is researched...
Additionally, armor makes absolute hull regen (such as that from a repair bay or repair ability) more "valuable"...if I have a ship with 200 hull and a hoshiko (which repair bots can repair 20 HP/s), that hoshiko is essentially regenerating 10% of the max HP per second...if a hull tech adds a 10% bonus to max HP, then the hoshiko would only be regenerating ~9% of the max HP per second (20/220)...
On the other hand, if I have gain 2 armor points, I in effect would have 220 hull...but the hoshiko is still regenerating 10% of my max HP...so, fixed hull regen rates are more meaningful for targets with higher armor since armor makes each individual HP point "last longer"...
The only advantage of hull techs is that they help passive regen (which armor does not help with)....however, in most circumstances the armor is going to be more valuable...