The following assumes that you are playing on difficulty 5. Naturally, what works on the highest difficulty will also work on the lower once.

A quick overview of what upgrade do:
Hull upgrades do two things - add turrets and increase hull hitpoints (moreso then other upgrades).
Wing upgrades increase agility and forward guns.
Engine upgrades do multiple things, but the main interest here is the reactor upgrades for faster shield and energy regen.

Turrets can only fire at what is in their firing arcs You cannot concentrate your lasers on one target like you can with forward guns.
Hull armor is surprisingly useless on difficulty 5. If your shields are completely down you get shredded very fast with or without the extra armor from upgrades. Shields are what keeps you alive.
With turrets it is easy to get lazy and die as a result. This is much less likely to happen with guns.
With turrets you have to point your flat top towards enemies to use them. Without turrets you can instead keep your flat side pointed at enemies, making you a harder target.
Turrets also can rob you of weapon energy at the worst time. With guns you can hold fire as necessary.

...So what are turrets good for?
Turrets are meant for attacking enemies that you can't get behind. Any damage is better then no damage.
Admittedly the Darkstar One is slightly less agile then every other fighter you will go up against,, but a few wing upgrades quickly fixes that.

Thus, if you are playing with Mesa Mod save hull upgrades for last, or if you are really curious only spend two upgrade points.
For the vanilla game? Don't spend any points in Hull at all.