The true value of shield upgrade is that it goes on all ships and negs a crit

Not quite. It CAN negate a crit but a lot of the time it just negates a hit. The HU lets the ship hold an extra damage card and there are times you don't want to do that but again you're looking at conditional events.

Previous debated between the Stealth Device and Shield Upgrade come up mixed because the SU costs that extra point and because the SD is so conditional. When it comes to comparing the Hull Upgrade and SU things are MUCH clearer:

HU: Ship takes one more damage card to destroy.

SU: Ship gets to negate one more [hit] or [crit] before taking cards.

If all you face are [hits] and face-down cards the HU is clearly superior as it costs 3 points vs. 4 points. Now [crits] and face-up cards can be a factor which the SU can better deal with but they need to happen at the right time but that "right time" probably doesn't happen enough to justify the added cost.

The true value of shield upgrade is that it goes on all ships and negs a crit

Not quite. It CAN negate a crit but a lot of the time it just negates a hit. The HU lets the ship hold an extra damage card and there are times you don't want to do that but again you're looking at conditional events.

Previous debated between the Stealth Device and Shield Upgrade come up mixed because the SU costs that extra point and because the SD is so conditional. When it comes to comparing the Hull Upgrade and SU things are MUCH clearer:

HU: Ship takes one more damage card to destroy.

SU: Ship gets to negate one more [hit] or [crit] before taking cards.

If all you face are [hits] and face-down cards the HU is clearly superior as it costs 3 points vs. 4 points. Now [crits] and face-up cards can be a factor which the SU can better deal with but they need to happen at the right time but that "right time" probably doesn't happen enough to justify the added cost.

side note if you face bombs or crew vader that also can cause some issues and may encourage a hull upgrade over shield upgrade

Actually, against crew Vader you want the SHIELD upgrade assuming its use hits the upgrade. When you use Vader it is just like you rolled an uncancelled [crit] except for the actually "hitting" part. If the ship has shields then the [crit] strips of a shield token just like a [hit] would. Having the SU means you get to negate Vader's effect once while the HU could be worse as you have another Hull to hold a face-up card. The Vader crew card DOES NOT ignore Shields.

Now getting caught b a Proton Bomb is a completely different story. The SU does nothing to protect from that while the HU can let you accept one additional card before being destroyed.